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When Aerith Gainsborough dies, put X +1/+1 counters on each legendary creature you control, where X is the number of +1/+1 counters on Aerith Gainsborough.
While vigilance is an evergreen mechanic, and so doesn't get specific support for decks in the same way mutate or ninjutsu might, there are still some interesting ways to use it
Also, and this is the main bit, you can exile it and the parts of Vecna to create an 8/8 Legendary Black Zombie God token with indestructible
double strike A creature with double strike deals damage twice each combat: once before creatures without first strike or double strike, and then again when creatures normally deal damage
A helpful blue spark will give you helpful advice and instructions as you follow the steps