Iascylla

What is the difference between Iascylla and Azir, Sovereign's uses of "this battlefield"?

Iascylla's delayed trigger refers to the battlefield that she held. Azir, Sovereign's attack trigger refers to the battlefield where he is located.

The difference between these two is that Iascylla has a delayed trigger that references implicit information from her trigger condition: the battlefield where she held. Azir, Sovereign's attack trigger does not reference information from his trigger condition, which purely checks for the event of Azir gaining the attacker designation for the first time each combat. Without that information being checked from the trigger condition, the "this battlefield" in Azir, Sovereign's attack trigger will be checked when the triggered ability resolves. If it resolves and he is not located at a battlefield, any instructions related to that battlefield will be ignored; otherwise, the battlefield he is currently located at will be used.

Comparatively, when a triggered ability refers to information from its trigger condition, that information is checked when the ability triggers and not on resolution. In the case of Iascylla, her delayed trigger is linked with her hold effect, so the information from the trigger condition of the hold effect can be shared with the delayed trigger.

In future sets, we've moved to use more precise templating and wording for abilities like Iascylla's.

Relevant Rules: 359.3.f.1., 359.3.f.2., 359.3.f.3., 359.3.f.3.b.

Akali is another case where her ability refers to information implicit to its own trigger condition, similar to Iascylla. Akali's "battlefield I moved to or from" doesn't require her to be on the board in order for us to be able to identify it, nor does it reference information about her current state. Even if she leaves the board, that information was checked when her ability triggered, and it is still available to us on resolution.

359.3.f.3.b. In the case of a linked ability that references information from a linked triggered ability, that information may be referenced from the trigger condition of the linked triggered ability if specified. Example: Iascylla reads "When I hold, at the start of your next Main Phase, you may move an enemy unit to this battlefield." The "this battlefield" in her delayed triggered ability refers to the battlefield she held, and so will be referenced from the trigger condition, when the triggered ability is generated.