First strike
When Grim Initiate dies, amass Zombies 1. (Put a +1/+1 counter on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
Army is a new creature type. It’s possible to control a nontoken Army (perhaps a creature with the changeling ability) and, through combinations of other cards, it’s possible to control multiple Army tokens. When instructed to amass, you may put +1/+1 counters on any of your Army creatures, and you may choose a different one each time.
If you don’t control an Army, the Zombie Army token that you create enters the battlefield as a 0/0 creature. Any abilities that trigger when a creature with a certain power enters the battlefield, such as that of Mentor of the Meek, will see the token enter as a 0/0 creature before it gets +1/+1 counters.