There will be spoilers, naturally.
The first apparition of the supposed "Forgotten God" is at the Vale of Shadows . There are several statues of "Him" there. Or perhaps they were Myrkul's , though I think that later in the game there is evidence that those aren't Myrkul's statues .
Afterwards you see statues of that same hooded figure at the Temple of the (guess who) Forgotten God . Well, if the god were Myrkul , then he would by no means be "forgotten" , right? So it must be someone else.
Poquelin , maybe? "He" may have started a cult in his name , and the statues bowed in prayer could actually belong to a fake Illmater priest. Also , Yxunomei's attack on the temple justifies it .
Is there any other evidence in the game? Perhaps more information about the rogue druid who stole the heartstone gem, or even Tolben, who is also a "hooded figure".