I never played it, but Google turns up a game called Maelstrom that seems to fit the bill.
Thank you! This is it. There isn't a lot of information online about this game (not even a Wikipedia article – though there is one for another 1992 game called "Maelstrom" that is unrelated). However, someone created an old-school fan website as a labor of love: http://www.overlordofharmony.com
I wonder how feasible it is to play an old DOS game like this on a modern Mac...
For the life of me I can't remember what this game was actually called. Does this ring a bell for anyone else? Would love to look it up again.