Amass

Amass is a keyword action introduced in War of the Spark, which creates a Zombie Army.

Description
Amass appears in because it is flavorfully connected to Bolas's army, the Dreadhorde.

As you amass, if you don't already control an Army creature, you create a 0/0 black Zombie Army creature token. Then put a number of +1/+1 counters on one of your Armies equal to the number after amass. If you do control an Army, you don't create any new tokens. Rather, you'll put those +1/+1 counters on an Army you control.

Amass was designed so you'd control one Army at most at any given time. But there are several ways around that. You could copy an Army you control, or maybe you control a creature with all creature types.

Rulings

 * 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.
 * Although players can respond to the spell or ability that instructs you to amass, once you start to amass, no one can interrupt you. This is true even if you're creating a new Army — no player can do anything while the Zombie Army creature token is still 0/0, before it gets the counters.
 * Even though the +1/+1 counters represent the growing numbers of Eternals joining your ranks, the Zombie Army creature token is still a single creature. It has two creature types: Zombie and Army.
 * Some spells that amass may require targets. If each target chosen is an illegal target as that spell tries to resolve, it doesn't resolve. You won't amass.
 * Some cards that cause you to amass also provide bonuses to "Zombie tokens." These affect any token that happens to be a Zombie, not just a Zombie Army you've amassed.