If Mirage Phalanx soulbounds with Aurelia, the Warleader can it produce infinite combats? Does the legend rule remove the old copies of Aurelia in time for the soulbonding with the new copy to happen?
The original Aurelia is still paired with Mirage Phalanx when the copy enters play. This is because the "legend rule" that destroys the original Aurelia is a state-based action (specifically, rule 704.5j). Rule 704.3 describes the timing of state-based actions:
704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event.
704.4 Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability.
The copy effect is resolved and the copy enters play before the active player gets priority. But soulbond requires the creature to be unpaired at the moment the other creature enters play:
702.95a ... “Whenever another creature enters the battlefield under your control, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control.”
So the full sequence of events is:
Mirage Phalanx's ability starts resolving, putting a token copy of Aurelia into play.
Mirage Phalanx's triggered soulbond ability sees the token enter the battlefield, but since Mirage Phalanx is still paired with the original Aurelia, it does not trigger.
The ability finishes resolving.
The active player (you) would gain priority, so the game checks for state-based actions. It sees the two Aurelias and removes one by the legend rule.
Only if you have Mirror Gallery or something similar in play.
Since Aurelia is a Legendary Creature, you'd normally only be able to have one in play at a time, thanks to the legend rule, so when Mriage Phalanx's ability triggers, you would then immediately choose to sacrifice either the original Aurelia or the token copy of her. However, cards that prevent this from occurring like Mirror Gallery exist; if you have one of these in play, then this wouldn't occur.
If the token copy of Aurelia persists, then you can potentially get infinite combat phases on your turn, since the Mirror Phalanx's ability triggers during the beginning of your combat phase, producing another Aurelia, and then one or both of the Aurelias attacks, triggering her ability and granting one (or two) additional combat phases and untapping all your creatures. Then, at the beginning of your next combat phase, Mirror Phalanx will trigger, creating another Aurelia, who can in turn create another combat phase when she attacks. This can repeat an indefinite number of times.
No, that does not work.
Here's the definition of soulbond, from the Comprehensive Rules:
702.95a Soulbond is a keyword that represents two triggered abilities. “Soulbond” means “When this creature enters the battlefield, if you control both this creature and another creature and both are unpaired, you may pair this creature with another unpaired creature you control for as long as both remain creatures on the battlefield under your control” and “Whenever another creature enters the battlefield under your control, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control.”
The second ability is the relevant one here. The requirement for both creatures to be unpaired is part of an “intervening ‘if’ clause”:
603.4. A triggered ability may read “When/Whenever/At [trigger event], if [condition], [effect].” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn’t true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the “intervening ‘if’ clause” rule. (The word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.)
Example: Felidar Sovereign reads, “At the beginning of your upkeep, if you have 40 or more life, you win the game.” Its controller’s life total is checked as that player’s upkeep begins. If that player has 39 or less life, the ability doesn’t trigger at all. If that player has 40 or more life, the ability triggers and goes on the stack. As the ability resolves, that player’s life total is checked again. If that player has 39 or less life at this time, the ability is removed from the stack and has no effect. If that player has 40 or more life at this time, the ability resolves and that player wins the game.
As that rule states, the condition is checked at the time the trigger event occurs (the creature entering the battlefield), while the copy-making triggered ability is resolving. At that time, Mirage Phalanx is still paired. The “legend rule”, being a state-based action, will be checked only after the copy-making triggered ability finishes resolving.
704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. Once no more state-based actions have been performed as the result of a check and no triggered abilities are waiting to be put on the stack, the appropriate player gets priority. This process also occurs during the cleanup step (see rule 514), except that if no state-based actions are performed as the result of the step’s first check and no triggered abilities are waiting to be put on the stack, then no player gets priority and the step ends.
704.4. Unlike triggered abilities, state-based actions pay no attention to what happens during the resolution of a spell or ability. Example: A player controls a creature with the ability “This creature’s power and toughness are each equal to the number of cards in your hand” and casts a spell whose effect is “Discard your hand, then draw seven cards.” The creature will temporarily have toughness 0 in the middle of the spell’s resolution but will be back up to toughness 7 when the spell finishes resolving. Thus the creature will survive when state-based actions are checked. In contrast, an ability that triggers when the player has no cards in hand goes on the stack after the spell resolves, because its trigger event happened during resolution.