What do we all know about the new member of the Arkati?
Personal theory from what I gathered.
It's a He.
It's a primordial Arkati slumbering before the Drake Wars. I would be so pissed if it's some Elven crap god.
It's evil. Um, duh?
It lures sailors, adventurers, refugees etc. from every region and race in order to feed off them.
It's primary domains are deception, secrets, lies, and vampirism. Awesome if this were all true, but one can dream.
One of the denizens on the Island is it's non-obvious avatar which I won't disclose due to spoilers.
It ultimately wants to finish a temple in order to enter our world. The typical super-villain plot pastiche.