Quote from chaikov »FYI
Strictly speaking, the ability triggers when the mutated cards hit the graveyard, at which time only the God-card actually has the ability.
However, an ability that trigger 'when a creature dies' is called a 'leaves-the-battlefield ability' (rule 603.6c), and this kind of ability has the privilege of 'looking back in time' to establish which cards are affected (rule 603.10a).
603.6c Leaves-the-battlefield abilities trigger when a permanent moves from the battlefield to another zone (…)
603.10a Some zone-change triggers look back in time. These are leaves-the-battlefield abilities, (…)