Succubus does have some very weird effects, including both glitches and secret(?) functions. It makes me wonder if there's something weird in the code that could also explain the secret power.
For example, succubus enemies often flip 180 degrees when hit or when summoning enemies on death (presumably some of them don't have mirrored animations or a mirror effect wasn't applied to them). Also, succubuses of any level appears to suck the shield off of the mini-bosses alongside the final boss, without actually converting them.
Maybe all of it is a payoff given that the succubus isn't allowed to use her powers on the boss and mini-bosses, but it would nice to have confirmed.