DarkPlaces is a game engine, used by Transfusion and Blood: Infuscomus, which is a heavily modified version of the Quake engine by id Software. Transfusion can also be run with other Quake engines, however it is primarily geared towards DarkPlaces. DarkPlaces is used in other major free software projects such as the original Nexuiz, its later forks Xonotic, Rexuiz, and Chaos Esque Anthology, content replacements such as Open Quartz, LibreQuake, and FortressOne, total conversions such as The Hunted and Blood Omnicide, as well as indie games Steel Storm, RetroBlazer, Wrath: Aeon of Ruin and DOOMBRINGER. It was programmed by Ashley Hale (LadyHavoc, formerly LordHavoc) and Transfusion is free software under the GNU General Public License. As a Quake engine, it mostly competes with Quakespasm/Spiked, vkQuake, and Ironwail, as well as the Kex Engine based 2021 official remaster.
As it is based on Quake, it is also largely compatible with the later id Tech engines. As such some maps from Transfusion have been converted to id tech 3 (Quake III engine) for Q3 BloodBath, while the mooted single player mode for Tranfusion was originally intended to utilize the Quake II engine under the title Caleb's Awakening. There is also a Caleb character model and bot available for Quake III (OpenArena compatible), made by Philip Stavitsky, as well as a Caleb player model for Kingpin: Life of Crime. Finally, there is also a fan fiction story by Kain-Xavier called "More Blood in the Arena Eternal" which creates a crossover between the storyline of Quake III and Blood II.