Etrigan is a demon, summoned by Merlin (the Merlin), and depending on which version of him you read, is either put into human host Jason Blood, or is Jason Blood.
There are tons of versions of him, and most depict him as huge, like a yellow bodybuilder on Super Soldier serum– but I think the most “realistic” (and scary) version of him was done in a 1987 4-issue mini-series called Demon by Matt Wagner and Art Nichols: the whole story essentially boils down to Etrigan’s attempt to rid himself of his human host, and takes place in Ireland and in several huge old libraries. It’s awesome, and even (from my limited knowledge of such) gets the details of spell-casting correct.
The best part is that Etrigan is just drawn as a thin (ish) yellow humanoid, which makes him even more eerie. (See above and below.)
I actually had a hard time finding information about this series (a comic book, with little information– ON THE INTERNET! Inconceivable!). The two coolest covers from the series are below.