So if Cratol moves to AC-11, he should not suffer an Attack of Opportunity from what I see. In that position would both Cratol and Tazskan have Advantage on attacks? Or does the Grapple negate that somehow? And my understanding of Grapple is that you can't Move, but you still can take your normal Attacks or Action, right? Because pouring damage onto the vampire might be more important than breaking free of the Grapple.
EDIT: I think regardless Cratol will suffer an AoO from the cultist at AA-11...
Edited: PaulNelson on 12th Dec, 2017 - 2:09am
Cratol would risk an AOO from the cultist at AA11. Also, to get the flanking advantage, he'd need to move further, such that he can reasonably be said to be on the opposite side of said vampire. Nearest square for that would be AC9, which would risk an additional AOO from the bat swarm.
If he does go to AC9 however, yes, he and Tazskan would both have advantage on her while all three stay in those relative positions. And yes, Taszkan is free to act, he just can't move of his own volition right now.
Drop the cultist, then go into the dining room. Yes the bats get an AoO, but next round Tazskan will get Advantage, and so will you. And if Tazskan Holds his action until you move into position, he gets it this round. The other cultists on your side of the hall are focused on Velon. On my side there are too many. If I drop one and go to help Tazskan, the others will get swamped.