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We have a Ranger and a Battlemaster in the party, and the Ranger thinks that he can benefit from the Colossus Slayer bonus to damage both during his Action and again later in that same round if the Battlemaster gives him an extra attack using Commander's Strike (which uses the Ranger's Reaction)...because Colossus Slayer only states "You can deal this extra damage only once per turn" and doesn't specify it has to be only on the Ranger's turn.

Since the Reaction is happening on the Battlemaster's turn, he reasons that Colossus Slayer is still only "once per turn" (it just happens to be on two different people's turns).

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