If you find Altar of the Loa and Plague on the same island, I assume you cannot sacrifice crew to the Altar before they would be eliminated via Plague because the effect of Plague occurs as part of the explore action and the Altar effect is optional and you wouldn’t be able to use it until after the explore action ends?
Plague has to be resolved before you can use the Altar, so your crew get killed, your ship gets kicked off the island, AND you have to leave the Altar behind. Frustrating combo to find!
If I eliminate crew to trigger the Altar and gain control of my opponent’s actions on their next turn, and then the Altar is eliminated later on my turn via the Kharmic Idol, does that mean the crew were sacrificed for nothing and the Altar effect is gone? I played it that it would still work on my opponent’s turn, because the crew had been eliminated and were gone for good (the Altar was already activated). I partly went that way because it felt more like a “trigger” effect unlike a persistent ability that is active throughout a round.
If the Altar is eliminated/removed before the controlled actions occur, they’re lost. It’s the same as if you used a sac-captain to eliminate a crew (or rolled an EA/SAT), but then it got cancelled or eliminated before you were able to take the bonus action.