Better known
(from time to time in his own series, even) as
The Demon - and occasionally as alter-ego
Jason Blood - Etrigan has been a quasi-mystical wild card unpredictably manifesting throughout
DC Comics continuity since 1972, when he was thought up by comics god
Jack Kirby.
Though the character's back-story and infernal heritage is occasionally intriguing (son of Raan Va Daath of the Pit and Belial - and through him, half-brother to the wizard Merlin) I find much more interesting Kirby's inspiration for his appearance, orange head and finny ears a direct emulation of a mask made from an upside-down tanned goose skin from a 1937 instalment of Hal Foster's Prince Valiant. Unfortunately this gives you a better understanding only into my research rather than of the character, so here's some backstory:
Back in the days of Camelot, Merlin attempted to utilize his half-brother as extra firepower against the forces of Morgan Le Fey. Unfortunately, he failed to keep Lovecraft's First Dictum in mind and, in order to prevent the demon Etrigan from wreaking havoc among foes and friends alike, was driven to bind the demon inside the body of an ordinary mortal man. Enter Jason. (Surprising absolutely no one, the "Blood" surname would be appended only after some misadventures with the demon newly inside him.)
Over the next thousand years Jason Blood, imbued with a strange longevity as a result of his baggage, researched religious and paranormal texts searching for ways to remove the demon from this world forever - a credible goal but it makes for boring comics. What you really need to know is that Jason changes fully into the Demon when the following chant is uttered:
"Change, change1, form of man,
rise the demon Etrigan!"
If you thought that rhyme was cute, you'll love the rest of the dialogue this badnasty spouts - to highlight the absurd nature of the monster's almost-constant over-the-top violence every word emerging from his lips comes in the form of silly, contrived couplets.
In recent years he's appeared as the Demon start to finish considerably more frequently than as Jason Blood, and though he usually has no running series of his own he pops up not infrequently to complicate the affairs of Batman, Dr. Fate, Hitman and the Blue Devil. An longstanding obscure fan favourite, he also played a major role in DC's 1999 "Day of Judgement" crossover and used his infernal powers to cross media, appearing in episode 010 of The New Batman Adventures: "The Demon Within," even logging an appearance back in the first few issues of The Sandman during Morpheus' first trip to Hell. For a goose-face, he certainly gets around.
1Pedant's note: some stories have the words "Change, change" substituted with "Gone, gone".