Malice: 23rd Century Ultraconversion for Quake is a commercial mod of Quake. While requiring a full, registered version of Quake, it acts like a stand-alone game, employing its own characters, weapons, environments, rules, etc. The player assumes the role of a mercenary named Damage, who works for the boss of an underground crime syndicate, Colonel Bossman. Damage's main mission is to assassinate the head of Takahiro Industries, Bossman's main rival. To do that, he has to venture into his heavily protected headquarters, dealing with security guards and robots on his way.

Set in the 23rd century in the year 2230, the player assumes the role of a bandana wearing mercenary named Damage. Working for Colonel Bossman and his underground crime syndicate, B. O. S. S. , the player is pitted against Bossman's main rival, Takahiro Industries. Takahiro Industries has its own security, too, with an army of guards as well as various sophisticated robots. It is these that Damage has to fight against, going through various futuristic environments in order to assassinate Takahiro himself. Being a total conversion of Quake, Malice features a similar gameplay style to that title. The game includes eighteen new levels, fourteen new enemies, various additional items (known as "Toyz" in the game), a new soundtrack, and new weapons. Unlike in Quake, weapons in Malice can be manually reloaded.