User blog comment:Zack Tan/Volume 7 Intermission/@comment-138.162.0.41-20150903140211/@comment-26932361-20150910005218

We don't exactly know Victim's ability but some people think it's a movement restriction seal, the players didn't specifically complain about Victim's ability they were complaining about the traps and other mechanisms in Nazarick being overpowered, and as the above commentor said Nifirea will never betray Ainz and Nazarick given that he is willing to risk his life in the fight in Vol.9 unless he was brainswashed or mind controlled or something, Ainz erased Enri's memories because he didn't want her to tell them that he was an undead in order not to scare them, but nonetheless Enri and Nemu despite being scared of Ainz to the point of wetting their pants, they still accepted him as their savior and thanked him just a few seconds later, so it would not be far-fetched that people might still accept Momon even after he reveals himself.

Edit: Actually we do know Victim's ability, which was mentioned in vol.4 that it is indeed a movement restriction.