Does the Dark Acolyte gain the power of the topmost unit in addition to its own power of 1, or is it replaced? If replaced, it seems rather weak, since there is a chance to get a unit with no power (Abyssal Terror).

Thanks.

When a card gains something, you always add it to what it already has. If a cards gains something from multiple sources, you add it all up. So Dark Acolyte has his printed power + the power of the topmost unit in the discard pile. If you play Warpstone Experiments on him, he has that + three power from the tactic, and so on.