[Nekros Immortal Shadow of the Dead minions]

Had the same happen to me just now. Was solo farming for the yellow toroids in the Temple of Profit, and as I was casting the Shadows, I got killed. I managed to summon one of the fallen enemies though, and once I clicked to revive, I noticed that one enemy I summoned was basically immortal. Didn't decay over time, was not on the ability counter or anything, just sort of floated there, attacked the mobs and that's it. Recasting the ability just summoned more guys, but this one was constantly floating in his spot and fighting on my side.