First strikeFirst strike is a static ability that modifies the rules for the combat damage step. (See rule 510, “Combat Damage Step.”)
If at least one attacking or blocking creature has first strike or double strike (see rule 702.4) as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
Giving first strike to a creature without it after combat damage has already been dealt in the first combat damage step won’t preclude that creature from assigning combat damage in the second combat damage step. Removing first strike from a creature after it has already dealt combat damage in the first combat damage step won’t allow it to also assign combat damage in the second combat damage step (unless the creature has double strike).
Multiple instances of first strike on the same creature are redundant. (CR 702.7)
When this creature
diesIs put into a graveyard from the battlefield. (CR 700.4),
amassTo amass [subtype] N means “If you don’t control an Army creature, create a 0/0 black [subtype] Army creature token. Choose an Army creature you control. Put N +1/+1 counters on that creature. If it isn’t a [subtype], it becomes a [subtype] in addition to its other types.”
A player “amassed” after the process described in rule 701.47a is complete, even if some or all of those actions were impossible.
The phrases “the Army you amassed” and “the amassed Army” refer to the creature you chose, whether or not it received counters.
Some older cards were printed with amass N without including a subtype. Those cards have received errata in the Oracle card reference so that they read “amass Zombies N.” (CR 701.47) Zombies 1. (Put a +1/+1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on an Army you control. It's also a Zombie. If you don't control an Army,
createTo create one or more tokens with certain characteristics, put the specified number of tokens with the specified characteristics onto the battlefield.
If a replacement effect applies to a token being created, that effect applies before considering any continuous effects that will modify the characteristics of that token. If a replacement effect applies to a token entering the battlefield, that effect applies after considering any continuous effects that will modify the characteristics of that token.
Previously, an effect that created tokens instructed a player to “put [those tokens] onto the battlefield.” Cards that were printed with that text have received errata in the Oracle card reference so they now “create” those tokens. (CR 701.7) a 0/0 black Zombie Army creature token first.)