Succubus Frontier

A mutated succubus crowns itself Demon King and starts converting every demon in sight into more succubi, a bloodless plan to breed humanity out of existence. One man decides that's not going to fly and gets up to do something about it, which is the whole plot in a nutshell.

The setup gives a clear villain and a clear reason to fight through her ranks, even if the specifics stay simple. Combat runs on turn-based encounters against a growing roster of monster girls, standing in for the succubus army the hero has to work through, with the usual mix of victories and losses along the way shaping how each fight plays out. It's a straightforward hunt-the-demon-king structure dressed up in fantasy trappings, not a branching epic.