User blog comment:B214/Talk About What You Want! 3/@comment-16281343-20150223194918/@comment-24502975-20150223234201

Hmm. Someone who is connected to Sona or her peerage and either has a grudge against Ise or wants to rematch him after being humiliated. It's too ominous to be Saji... I really can't think of anyone :|

The only time anyone from Sitri would have been humiliated is when Ise first revealed Bilingual, but obviously based on interactions in volume 14 and onwards no one is holding a grudge towards him for it. On the contrary, they're good friends.

To make a challenge or threat of that level against the Gremory group, they'd have to be pretty powerful... Maybe it's related to Rugal? I don't see any reason for discussing a change of tactics due to a second Rook unless they had another Rating Game though.