The Eternity TC is an abandoned total conversion by Team Eternity with many advanced features planned, necessitating the use of a custom engine. While work on the TC itself ceased ca. 2003, work on its custom engine continued, shedding the features that had been added to it that were too specific to the Eternity TC.
The progression used the typical three-episode structure, first a first episode happening on Earth in the UAC's "Terran Temporal Research Facilities", a second episode in the Hexen-inspired fantasy setting of "Halcyon", concluding as traditional with a third episode in Hell.
The Eternity TC was centered on extending the helper dog mechanism to turn it into a generic helper AI system, with different types of sidekicks having different behaviors and abilities. Such sidekicks included a fellow space marine named "John 'Carnage' Pietrovich" from the first episode and a dwarf wizard named "Halif Swordsmythe". Halif remained the mascot of the Eternity engine for a while.
The TC also planned new weapons, a spellcasting system, and several new enemies. Of those, one, the arachnophyte, found its way to the Monster Resource WAD and has been used in several other projects since.