415.5. Killing can be attributed to one or more Game Objects.
415.5.a. The Killed Unit or Gear is said to be Killed by that Game Object.
415.5.b. A spell or ability that contains a Kill instruction is responsible for Killing the Unit or Gear.
415.5.c. When one or more Units is killed due to a Cleanup , the spell or ability that resolved immediately prior to that Cleanup that applied damage to the Unit or Units is responsible for that Kill action.
415.5.d. Abilities originating from Game Objects that are responsible for Kill Actions are responsible in addition to the Game Object that created them. Example: Falling Star is a spell that says “Do this twice: Deal 3 to a unit.” Immortal Phoenix is a unit that says “When you kill a unit with a spell, you may pay [1][C] to play me from your trash.” A player plays Falling Star while Immortal Phoenix is in their trash. The “do this” phrasing on Falling Star means that it has a reflexive triggered ability, which places two triggered abilities on the chain. As each of those triggered abilities resolve, it deals damage to the unit chosen for that ability. If one of these abilities deals lethal damage to a unit, both Falling Star and its ability are considered sources of the damage, and so both Falling Star and its ability are responsible for killing the unit. This means that Falling Star’s controller killed a unit with a spell, so Immortal Phoenix’s ability will trigger.