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All Will Be One's ability will trigger any time you put one or more counters on a permanent or player. This might be due to a spell or ability resolving, a permanent you control entering the battlefield with counters, or combat damage from a source with toxic, infect, or wither.

If more than one creature with toxic deals combat damage to a player at the same time, those counters are placed as a single event, and the ability triggers one time.