Yes, you would need to complete the power or action. Your situation would be a Sacrifice Victory, meaning you both win and lose. See FAQ here:
What creates a sacrifice victory? What do you finish evaluating before deciding whether you have won or lost?
You finish evaluating the current effect, for instance a Power or a Ravage in a single land. If that triggers both a win condition and a loss condition, it is a sacrifice victory (p. 12).
For instance, if Pillar of Living Flame does enough damage to destroy the last needed Invader and then adds the last Blight to the island, that is a sacrifice victory. If a Ravage in a land adds the last Blight, but then the Dahan retaliate and wipe out the last Invaders for the current victory condition, that is also a sacrifice victory.
But the way I see it, you managed to drive off those pesky Invaders. And at least, you did not sink the whole island into the ocean doing so.