It's not an April Fool's day thing, as developer Barney Cumming pointed out in the comments section, and he's aiming for the new boss to be introduced in the next update - though there's no timeframe on when that will hit right now.
Valve's boss and the Lord of All PC, or something, Gabe Newell is a large boss character in multiplayer dungeon-'em-up Crawl.
A hypnotic gif of Gaben was posted over on developer Powerhoof's site. Seriously, I can't stop watching it.
The best bit is, of course, the fact that when he powers up he grows a beard and pulls his knivesout. Oh, Gaben.
We had a lookat Crawl when it was in an even earlier state some months ago, but even then it was great fun.