Rule 429.5.c

When one or more Units is killed due to a Cleanup, that kill action is attributed to the spell or ability that resolved immediately prior to that Cleanup that dealt damage to the Unit or Units.

• 429.5.c.1 The player responsible for the deal action is responsible for the kill action.
• 429.5.c.2 If the Cleanup that caused the units to be killed was the Combat Cleanup, the sources of the Combat Damage are…

Rulings that cite this rule

• What does it mean to “kill a unit with a spell?” It seems like Hidden Blade counts, but what about a spell that deals damage? A spell that causes a unit to deal damage? A spell that causes a unit to die in some other way, like Stupefy on a unit that has 1 more Might than it has damage currently marked on it?

Official Core Rules