If Azir, Sovereign moves into a battlefield and his ability goes on the chain, but the opponent uses Fate of the Fallen (FoF) in response to move him back to base, does his ability fail because 'this battlefield' is now NULL?

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Accepted Answer

No — the triggered ability does not automatically fail just because Azir, Sovereign was moved back to your Base. A triggered ability stores the contextual information it needs (like which battlefield was “this battlefield”) when it triggers, and will attempt to resolve using that stored information. However, individual instructions that require targets at that battlefield can still fail or be ignored if those targets are no longer legal by resolution [359.3.f.4] [359.3.e.5] [359.3.e.9].

When Azir, Sovereign moves into the battlefield and his ability triggers, the triggered ability keeps the contextual reference to that battlefield as part of the trigger. If the opponent plays Fate of the Fallen (or any effect) in response and moves Azir, Sovereign to your Base, the triggered ability itself does not automatically become NULL merely because Azir, Sovereign changed zones. The ability will resolve and reference the stored battlefield information from the trigger [359.3.f.4]. If the ability’s instructions require a target that must be at that battlefield, and that required target no longer exists or is otherwise illegal by the time the instruction executes, that instruction will be ignored or will mistarget per the invalid-target rules [359.3.e.5] [359.3.e.9] [359.3.e.7]. If the ability instead depends on Azir, Sovereign specifically being present (for example, if it specifically targets or references the unit as a target and the unit has left to a Non-Board Zone and returned such that it is no longer the same object), then that part may fail under the Non-Board Zone / identity rules [359.3.e.4].

Azir moves to a battlefield; his triggered ability triggers and is put on the chain, recording the battlefield context. Opponent reacts with Fate of the Fallen and moves Azir, Sovereign to the Base (a Non-Board Zone). When Azir’s triggered ability resolves, it uses the information stored on the triggered ability (including which battlefield was “this battlefield”) to execute its instructions [359.3.f.4]. For each instruction that needs a target at that battlefield, check whether the target is still legal at resolution. If a target is no longer legal, that instruction does nothing for that target [359.3.e.5] [359.3.e.9] [359.3.e.7]. If the ability explicitly required Azir himself to be a legal target and he is in a Non-Board Zone (or has returned and is treated as a new object), that target may be invalid under the Non-Board Zone identity rules [359.3.e.4].

If the printed text of Azir, Sovereign makes choices at trigger time vs. at resolution (see [FAQ:unleashed#3] for the general principle of when decisions occur), that can change what is stored on the trigger and what can be re-chosen when it resolves. If Fate of the Fallen also gains control of the triggered ability or allows new choices to be made for it, that can deliberately change what objects are "enemy/friendly" to that trigger and potentially cause mistargeting as in the Yasuo example [359.3.f.4]. If Azir, Sovereign is moved to a Non-Board Zone and then later returned to the battlefield before resolution, the returned object may not be the same object for targeting purposes and could be an invalid target per [359.3.e.4].

When a unit moves and a triggered ability is put on the chain, the trigger carries with it the contextual information it needs to resolve. The Core Rules explicitly say that some information used by a triggered ability (for example “enemy” / “friendly” status) is referenced from the triggered ability itself and is checked when the ability executes, not dynamically replaced by later changes to the permanent that triggered it [359.3.f.4]. By analogy, the battlefield referenced by a trigger when Azir, Sovereign moved there will be the battlefield the ability remembers when it resolves. That means the ability does not automatically become NULL simply because Azir, Sovereign was moved back to Base in response. However, the actual instructions of the ability still must be executed with legal targets. The rules for illegal/mistargeted instructions make clear that if a required target or required condition is no longer valid by resolution, that instruction will be ignored or fail (examples and rules: [359.3.e.5], [359.3.e.9], and [359.3.e.7]). Additionally, moving a permanent to or from a Non-Board Zone and then back can cause it to be considered a different object for targeting purposes, which can make references to that specific object invalid [359.3.e.4]. Finally, if the precise card texts cause choices to be made at trigger time vs resolution (or allow a player to take control of the triggered ability or make new choices for it), those specifics can change the result — the Yasuo example in the rules shows how gaining control of a triggered ability or making new choices can lead to different outcomes [359.3.f.4]. Given the lack of the two cards’ printed texts in the provided context, this is the most defensible, rules-based interpretation: the trigger itself survives the move and will try to resolve using its stored battlefield reference, but any parts that require targets at that battlefield can fail if the targets are gone or illegal by resolution.