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.
Treason of Isengard
You can cast Treason of Isengard without a target just to amass Orcs. However, if you do choose a target, and that target is illegal at the time Treason of Isengard tries to resolve, Treason of Isengard won't resolve and none of its effects will happen. You won't amass Orcs.
In the rare case that you control multiple Army creatures (perhaps because you played a creature with changeling) while you amass Orcs, you choose which of your Army creatures to put the +1/+1 counters on. If that creature isn't an Orc, it becomes an Orc in addition to its other types.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Pavel Kolomeyets
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