Heretic guy will give you a key if you ask about it as already stated
and he will tell you the way to the back entrance of The Serpent Temple.
Go head and park your horse, you will find another one on the west side of Ishad N'ha and I know where Mammaw LOL but I ain't tellin!
I always used the rocky edge of a path near Valeia to park the horse and raft so I could trade one for the other when traversing back and forth for quests...you just have to leave the raft at the opening of the caves and swim the length to Ishad N'ha is all.
Glad you are enjoying your gaming! I have been reading the updates on your adventures and am rooting for you!
Hugs!
Moni