Harvald

He is full of rats and is older than recorded time and was sealed away as punishment

for trying to eat other gods. He's been in an... incredibly ancient interdimensional library of sorts?

And he was used as a guardian for that library - And the isolation kinda rewired his brain and he doesn't socialize well :)

He WILL eat anyone who tries to access the library/find the library

because he is starving and hasn't seen other life forms in such a long time :)

The Old Gods have a very ancient interdimensional library where they store all the knowledge and secrets, and they can't just let all this knowledge fall into the wrong hands [it's like the source code for the universe basically or something].

So they chain/bind him to the library, so even if there is a breach, he cannot escape unless he has help on the outside! It's not like he can do anything with the information, anyways :>

Also He CAN speak human languages, It is just an active conscious effort every time he does,

and mainly communicates via staring judgmentally into your soul (or just looking at you like an afternoon snack),

strange clicking sounds, hissing, incomprehensible and discordant noise pollution, and growling 

His real self is an amorphous mass that sometimes produces eyes or random limbs and body parts in some kind of sick mockery of an attempt at other lifeforms, And his human disguise/form may or may not be borrowed from some poor fool he devoured ages ago!