It's an "Everything you know is wrong" story, but this was the time to do it.

We've seen a number of Demon appearances here over the years, and in them his level of badness has varied from Chaotic Good to eat-a-baby, full-on Belichick evil. Today the knobs go up to 11. From 1994, the Demon #0, "Zero Hour" Written by Garth Ennis, art by John McCrea. Despite the title and the tie-in, this didn't have much else to do with the crossover...

Jason Blood has just drank an elixir prepared by himself and Merlin, removing the mental blocks Etrigan had placed on Blood. Having eleven centuries of memories suddenly return would probably be bad enough, but eleven centuries of his crimes and sins may be more than his sanity can bear. In the year 560, Jason was a family man, with no interest in the possible fall of Camelot; but Merlin hears his grousing as treason. Likewise, Merlin's half-brother Etrigan has not been a great deal of help against Morgaine Le Fey, and Merlin knows he'll soon be unable to control the ambitious Demon. So, Merlin kills two birds with one stone, trapping Etrigan in Jason; driving both of them completely insane. Jason kills his family, then the Demon murders his entire village; earning them the name Jason-of-the-Blood.

Since Jason had been a good, if cynical, man; Etrigan has work on him to make a good host. He alters Jason's thoughts and memories to make him more amenable; making Jason a willing partner. Using Etrigan's power, Jason quickly amasses a massive fortune; but Etrigan's also making moves to keep advancing in the hierarchy of hell. After the brutal murder of a cardinal, Jason confronts Etrigan, and tells him no more; Etrigan alters his mind again. After more centuries of atrocities, Etrigan and Blood are invited to Hell to see Lucifer. (Described here as "a million miles away from the flake who will eventually quit his post.") Blood asks Lucifer where to go for new experiences and opportunities, and is told simply "west." Blood and Etrigan go to America, and commit about every horrible thing ever done in this country; until in July 1917, at a party Jason almost literally says "hold my beer" and leaves to wrap up the first World War. (Partly because he could, mostly for a million gold sovereigns.)

On August 1, 1917; Jason Blood is moved to save "a regiment of virgins," sad, doomed soldiers; and unleashes the Demon against the Germans. And the Demon even brings back-up, partly because he has advanced in rank to rhymer, partly because he felt Jason's deal didn't really do anything for him. After slaughtering the Germans, he does the same to the English; leaving ten thousand dead in the trenches. Jason loses his mind again, and the Demon is forced to erase his memory, but goes too far: everything from when Merlin joined them is forgotten. Jason believed himself to be a good man, struggling to get separated from Etrigan; piecing together clues over almost eight decades. Now, with Etrigan planning to unleash his son on Glenda and her unborn child, Jason has forced Merlin to show him the truth. (And in truth, Merlin was responsible for much of it!) All that saves Merlin, is that Jason needs Merlin's help to kill Etrigan...!

There was less than a year left of this book, before Ennis and McCrea moved on to Hitman, but I don't think the Jason vs. Etrigan plot ran the whole distance. Still, not unlike Marvel's Ghost Rider, every so often the Demon is portrayed as evil who happens to occasionally take out other evil, but not necessarily good...