Gríma Wormtongue

Some cards refer to the "amassed Army." That means the Army creature you chose to receive counters, even if no counters were placed on it for some reason.

Some spells and abilities that amass Orcs may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't amass Orcs.

Amass Zombies works the same way, except you create a 0/0 black Zombie Army creature token if you don't control an Army. If the Army creature you chose isn't already a Zombie, it becomes a Zombie in addition to its other types. By combining cards with amass Orcs and amass Zombies, you can end up with an Orc Zombie Army.