The way I portray him, he's a being of class and advanced vocabulary. Aside from dialog, ways to describe him in the story can be as if you were describing a horrific event or sight. Give intimidating details for an intimidating mental image. If you want to give him some more physically noticeable emotion, you can say "his face was sunken in slightly, the shadows of his face gave him a look of *insert emotion here*. But I'd use that for if he were angry. I also like to think his face slowly rips open to form a "mouth" and makes gruesome, skin crawling sounds as it does.

And one more thing about tentacles, I've always wondered if things would be more scary if they were actually branches. Black, cold, constricting branches.

But that's if you want to portray him in a scary way.