Chillax is a megawad compiled by an anonymous author and noted for its extreme difficulty. Most, perhaps all of the maps are plagiarized from other WADs, with monsters liberally added in. The mapset is often played on ZDaemon servers and is meant for cooperative play, though some of its maps have also been speedrunned in single-player mode.
The megawad was originally named "phmlspdst-v1.x" and contained modified speedmaps by Phml, taken from his WAD of the same name. While Phml was not opposed to his maps being copied (as he gave permission to reuse and modify maps in their textfiles), he was upset at the title, which implied that the modified Phmlspd mapset was officially condoned by him, and protested on the Skulltag forums, which led to the megawad being quietly retitled. 
1. ^ MAP30 is divided into eight separate sections, the first seven of which are shaped such that they spell "CHILLAX" on the automap. These are based on areas from previous maps in the WAD.
Jace Hall used this mapset with the "Hard Doom" monster replacer and the Zandronum port to organize a challenge, entitled "Doom Arena", for interested players; with monetary rewards for those managing to complete a level.