ONE CASE OUT OF THREE

The combat phase runs beginning of combat -> declare attackers -> declare blockers -> combat damage -> end of combat. Nothing is on the stack yet; the first thing that happens is the beginning of combat step.

Commander's default multiplayer setup is Free-for-All with the attack multiple players option, so unless the table has agreed otherwise that is the configuration in force here. A Free-for-All table must use exactly one of attack left, attack right, or attack multiple players. Under attack multiple players — the Commander default — NO turn-based choice is made in this step: every opponent is already a defending player, and you choose which player each creature attacks individually as you declare it. 506.2a's turn-based choice applies only to the other branch, and even there it is scoped: 506.2a covers the case where the attacking player chooses, and it expressly notes that the choice may instead be DICTATED by the variant or option in use — which is what attack left and attack right do (they hand you a fixed defending player, you do not pick one). So it is wrong to say 'either way the choice is a turn-based action you make'; under attack multiple players there is no such choice, and under attack left/right the choice is not yours. What IS true on every branch is that the determination is made fresh in EVERY combat phase, because each additional combat phase Najeela creates has its own beginning of combat step and its own declare attackers step. Under the Commander default that is what later licenses pointing the army at opponent B (step 22); under attack left/right the redirect in step 22 would NOT be available, because the defending player would be fixed by seating.

Two creatures are declared as attackers. Exactly ONE of them is a Warrior: Najeela is a Human Warrior; Derevi is a Bird Wizard. Declaring them taps them (neither has vigilance). Both were legal choices: untapped and controlled since the turn began. Each attacker's defending player is chosen here, per creature.

“Whenever a Warrior attacks” triggers per Warrior declared as an attacker. Najeela is the only Warrior declared, so exactly 1 trigger. Derevi (Bird Wizard) is not a Warrior and generates nothing. Trigger count, combat 1 = 1.

Token A is put onto the battlefield already tapped and attacking; since Najeela's ability does not name a defending player, you choose one as it enters — opponent A. Attacking creatures in combat 1 = 3 (Najeela, Derevi, token A). Total power on the table = 3 + 2 + 1 = 6.

A creature put onto the battlefield attacking was never declared as an attacker, so for trigger purposes it never “attacked.” Najeela's “Whenever a Warrior attacks” therefore does not see it. It IS an attacking creature for every other purpose — it deals combat damage (step 8), it is untapped by Najeela's activated ability (step 12) and it can be declared again next combat (step 15). The tokens simply do not chain within a single combat: 1 Warrior declared -> 1 trigger -> 1 token, full stop. (This is what keeps the loop finite per combat instead of infinite inside combat 1.)

Blocking is a choice the DEFENDING player makes from among creatures they control; opponent A controls no creatures, so there is nothing they can choose and no block can be declared. Every attacker therefore stays unblocked, and each unblocked attacker assigns its combat damage to the player it is attacking. This is the standing assumption for every combat in this line (the scenario stipulates the opponents have no blockers).

Opponent A: 40 -> 34. No lifelink yet (Najeela's activated ability has not been used), so you gain nothing from this damage.

Combat damage is dealt simultaneously, but that single event contains three separate occurrences of “a creature you control deals combat damage to a player,” so the ability triggers once per creature. The three triggers are put on the stack the next time a player would receive priority, each choosing a target permanent (you target your own lands). Derevi trigger count, combat 1 = 3 = number of attacking creatures.

Announcing the ability puts it on the stack; activating an ability then follows the casting steps, and the last of those steps is paying the total cost. A player may activate mana abilities while paying that cost — that is precisely when a mana payment is being asked for — and those mana abilities do not use the stack, so the five mana appear in your pool and the {W}{U}{B}{R}{G} is paid in full. The important consequence of taking line L2 is that you do NOT have priority at the moment the lands are tapped: priority is only handed back after a mana ability when the player HAD priority before activating it, and during cost payment you did not. Najeela's ability is therefore put on the stack ABOVE the three Derevi triggers — but it is NOT the last thing to go on the stack, because the City of Brass trigger produced by the cost payment is still waiting (step 11). Resolution order will be: City of Brass trigger, then Najeela's ability, then the three Derevi triggers. Mana Confluence's “Pay 1 life” is likewise part of the activation cost of its mana ability, not a trigger — it never uses the stack.

This is the L2 bookkeeping, stated explicitly because the two lines order the stack differently. Under L2 you were paying a cost when City of Brass became tapped, so you did not have priority; 117.3c (which returns priority to a player who HAD priority when they activated an ability) applies to your activation of NAJEELA's ability, not to the land taps, and 405.6c does not hand priority back after the mana abilities either. A triggered ability waits and is put on the stack the next time a player would receive priority — here, the moment you receive priority after finishing the activation — and it becomes the topmost object on the stack. Najeela's ability is already on the stack by then, so the City of Brass trigger sits ABOVE it and resolves FIRST, dealing you 1 damage (you: 40 → 39); Najeela's ability resolves next (step 12), then the three Derevi triggers (step 13). (The alternative line L1 — tap the lands while holding priority, float the mana, then activate Najeela — is equally legal but produces the OPPOSITE stack order: the City of Brass trigger would be put on the stack beneath Najeela's ability and Najeela's ability would resolve first. Either way 1 damage is dealt and nothing downstream changes, but this resolution commits to L2 so the step order 11 → 12 → 13 is the true resolution order.) The trigger is NOT a mana ability: it cannot add mana to a mana pool, so it uses the stack like any other trigger. Every tap of City of Brass puts another copy of this trigger on the stack, including the repeat taps inside a single damage step described in step 17. Mana Confluence is the contrast case: its life payment is a cost, so it produces no stack object.

Najeela, Derevi and token A are untapped and now have haste (critical: token A was created this turn and could not otherwise be declared as an attacker) and lifelink. Token A is a legal object for “all attacking creatures” because it is an attacking creature (step 6) even though it never “attacked.” The untap is NOT self-defeating: untapping a creature that has already been declared as an attacker does not remove it from combat and does not prevent its combat damage, so all three remain attacking creatures and remain able to be untapped/redeclared without losing anything. The extra combat phase is added directly after this combat phase.

Combat 1's triggers alone cannot refund the activation; the first activation was funded by the five lands that were already untapped before combat. Ceiling for the step is 5 taps + 3 untaps = 8 mana, of which 5 were spent. You end the step with 3 untapped lands. Unspent mana is then lost as the step ends. NOTE: steps 10–13 walk the NAIVE line — tap all five lands at once, which costs 1 damage to City of Brass and 1 life to Mana Confluence. Step 23 shows a strictly better line on this same board that pays 0 life and still leaves 3 untapped lands, so the 2 life spent here is a choice, not a floor.

“Attacking” does not carry over. Every creature must be declared as an attacker again in the new declare attackers step — which is exactly why the token pool converts into new Najeela triggers each combat — and the defending player is determined again (step 2).

Token A is legal to declare because Najeela's ability untapped it and gave it haste. Tokens B and C were put onto the battlefield attacking, so they generate no triggers this combat. Attacking creatures, combat 2 = 5 (Najeela, Derevi, A, B, C).

Derevi trigger count equals the number of your creatures that dealt combat damage to a player = 5. Tokens B and C do not have lifelink yet (they were created after the grant), so the lifelink cohort lags the damage by one combat.

Each of the N triggers is therefore worth up to one extra mana within the one damage step, without ever needing more than one land: 5 triggers = up to 5 extra mana = exactly Najeela's {W}{U}{B}{R}{G}. So FROM COMBAT 2 ONWARD THE LOOP PAYS FOR ITSELF with no lands carried in (in fact 3 untapped lands are carried in from combat 1, giving 3 + 5 = 8 available against a 5 cost, leaving 3 untapped again). The interleave is legal because the stack resolves one object at a time and the active player gets priority after each resolution — you are never forced to let the whole stack empty first. Second activation -> untap all attackers, grant trample/lifelink/haste (now including B and C), add combat phase 3.

Combat damage = 3 + 2 + 1x7 = 12 (opponent A: 26 -> 14); life gained from the 5 lifelinkers = 8. Derevi triggers = 9, against a fixed 5-mana cost: a surplus of 4 untaps even before counting the 3 lands carried in. The loop is now strictly growing, not merely break-even.

Triggers first reach the 5-mana cost in COMBAT 2 (5 = 5, break-even) and exceed it from COMBAT 3 (9 > 5). Combat 1 is the only combat that must be funded from outside the loop (3 triggers < 5), and it is funded by the five lands untapped before combat. The loop is therefore self-sustaining from combat 2 and runaway from combat 3.

Nobody gets priority in between: state-based actions are performed the moment a player would receive priority, before that player actually does, so opponent A leaves the game before you could respond and before the Derevi and City of Brass triggers from that damage are put on the stack. Trample is irrelevant here (no blockers) but makes blocks non-answers.

506.4's list of removal causes is exhaustive and the defending player leaving the game is not on it: a permanent is removed from combat only if it leaves the battlefield, changes controller, phases out, is specifically removed by an effect, is an attacked planeswalker/battle that stops being one, or is an attacking/blocking creature that regenerates, stops being a creature, or becomes a battle. None applies. 508.1k keeps each declared attacker an attacking creature until it is removed from combat or the combat phase ends, and the multiplayer rules for a departing player do not add a removal: 800.4e only stops future combat damage assignment (irrelevant — this combat's damage is already dealt), and 800.4a/800.4b only touch objects owned or controlled by the departed player. Your tokens are yours, so 800.4b's “no token is created” clause never bites. So: Derevi's 17 triggers and the City of Brass triggers go on the stack, you interleave taps and untaps (step 17), pay {W}{U}{B}{R}{G}, and Najeela's ability untaps all 17 attackers, grants haste, and adds combat phase 5. Had removal occurred, the untap would have found nothing, the 8 tokens made this combat would be hasteless, and combat 5 would be empty — the line works precisely because removal does not occur.

COUNT, precisely: 17 creatures are DECLARED as attackers. 16 of them are Warriors (Najeela plus the 15 tokens; Derevi is a Bird Wizard), so Najeela triggers 16 times and 16 new tokens are put onto the battlefield tapped and attacking. Those 16 tokens are NOT declared attackers, so 508.1a's untapped/haste requirement never applies to them; they are attacking because they were put onto the battlefield attacking, and their controller chooses the defending player as they enter — opponent B. That gives 33 attacking creatures in combat 5 (17 declared + 16 put onto the battlefield attacking), which is the 33 of the A' = 2A - 1 recurrence. The 17 DECLARED creatures are each legal to declare because Najeela's combat-4 activation untapped them and gave them haste until end of turn. Redirecting at opponent B is sound because the Commander default is the attack multiple players option (step 2): all opponents are defending players and you choose the defending player per creature as you declare it. Under attack left or attack right this redirect would be unavailable — there the defending player is dictated by the option rather than chosen. Combat 5 deals 33 + 3 = 36 to opponent B (40 → 4); combat 6 declares 33, adds 32 tokens for 65 attackers and 68 damage, and kills them.

The set of actions can be repeated indefinitely and each iteration strictly increases attackers, damage and untaps, so no external resource is consumed faster than it is replenished. The only candidate drains are life: 1 damage per City of Brass tap (a triggered ability that uses the stack, once per tap — step 11) and 1 life per Mana Confluence tap (a cost). BOTH ARE AVOIDABLE ENTIRELY on the audited board of step 24. City of Brass's ability triggers only when that land becomes tapped, and Mana Confluence's life is paid only when its mana ability is activated — and activating a mana ability is never mandatory. So if you never tap those two lands, you never pay anything. You never have to: Command Tower produces one mana of any color in your commander's color identity, and this deck's commander is Sisay, Weatherlight Captain, whose color identity is all five colors, so Command Tower alone covers {B} — the one colour Plaza of Heroes cannot make. Plaza of Heroes' third mode makes one mana of any color among legendary permanents you control; the legendary permanents you control are Najeela (red) and Derevi (green/white/blue), an object being the colour of the mana symbols in its mana cost, so Plaza covers W, U, R and G with no spend restriction. The line: with the Derevi triggers on the stack, tap Plaza for {W}; pass; a Derevi trigger untaps Plaza; tap it for {U}; pass; a second trigger untaps it; tap it for {R}; pass; a third trigger untaps it; tap it for {G}; then tap Command Tower for {B}. Five taps, three untaps, {W}{U}{B}{R}{G} floating, all inside one combat damage step so the pool never empties — and neither City of Brass nor Mana Confluence was ever tapped. Combat 1 supplies EXACTLY the three Derevi triggers this needs; every later combat supplies five or more. The floor is therefore 0 life per combat, in every combat of the loop. The 2 life spent in steps 10–13 is the price of the naive tap-all-five line, not a lower bound; and even that is dwarfed by lifelink gains of 6, 8, 12 ... from combat 2 on. End state: Plaza and Command Tower tapped, City of Brass, Mana Confluence and Exotic Orchard untapped — the same 3 untapped lands the naive line leaves.

Precise requirements: (1) five mana of five DIFFERENT colors every single combat, re-produced each time because mana pools empty each step; (2) mana permanents that Derevi's untaps can recycle — Derevi untaps one target permanent per trigger and only tapped permanents can be untapped, so the untaps are worth mana only if you control sources covering W, U, B, R and G; (3) at least 5 of your creatures connecting with a player each combat (combat 1 yields only 3 triggers, so the first activation must be pre-funded); (4) Najeela on the battlefield at each declare-attackers and Derevi on the battlefield at each combat damage step. BOARD AUDIT, as literally listed: opponents control no lands, so Exotic Orchard produces nothing; Plaza of Heroes cannot produce {B}, because its any-color mode is restricted to casting legendary spells and so cannot pay an activation cost, and its third mode is limited to colours among legendary permanents you control — Najeela (red) and Derevi (green/white/blue), i.e. W, U, R, G. That leaves four usable lands: Command Tower, which makes any colour in Sisay's five-colour identity and therefore covers {B}; Plaza, covering W/U/R/G; and City of Brass and Mana Confluence, which each make any colour at a cost of 1 life. CONCLUSION ON THE STIPULATION: Command Tower plus Plaza ALONE cover {W}{U}{B}{R}{G} in five taps requiring three Plaza untaps, and combat 1 supplies exactly three Derevi triggers while every later combat supplies five or more (step 23). City of Brass and Mana Confluence therefore never have to be tapped at all. The scenario's stipulation that all five lands make any of WUBRG is consequently not load-bearing in any respect — it does not enable the loop and it does not even change the life paid, because the true minimum is 0 per combat on the audited board and 0 per combat under the stipulation alike.