You will need an List for this.
Lists are like arrays, but are mutable, meaning you can add and subtract from them running runtime.
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The List should be on an object that’s in the game all the time, either an actor or game mode.
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Once the player activates a trigger,
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it activates the barrier
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spawn the AI, with a BeginPlay that adds itself to the List by calling the List’s Custom Event
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Once the AI dies, it calls a different Custom Event on the List. That Event should evaluate every time it’s called if the List size is <= 0. If so, then deactivate the barrier.
Simple really.