From what I have seen before R'lyeh got a new player [and got some stales], Marignon got a chance to grab some cheap lands and grab a nice bunch of R'lyeh provs. I guess Marignon hoped that R'lyeh will be set AI or new player won't be so competent?
Ah.. no. I didnt attack Ryleth at any time, and before the old player left I've told him to take all my water provinces to kill off the remaining Atlantian forces.
I counted on further teamwork, actually. Thats why I'm of course rather pissed now