I have a prisoner and I have no clue how it's working right now. Sometimes red x's, sometimes not, sometimes only on one team. I've stunned a sentinel harpy even when she didn't have a red x (or a prisoner on her team). That part is probably still bugged.
I know sentinel harpy is. She holds on to beneficial effects for an extra turn sometimes. I use her and my AGS for farming LOL and she'll usually keep the void shield after everyone else has dropped theirs. Also, in pvp my prisoner stunned one (with the red x) on defense, he also gave her back the silence icon. She promptly used her power ability to cleanse herself and then attack back. Sadly, the one I have on defense doesn't seem to know that trick.





