UX-73 was the designation of a prison planetoid in the Negative Zone, where slaves mined materials to build ships and weapons for the Annihilation Wave, including the Harvester of Sorrows.

In their search for a way to stop the Harvester, Iron Man, Mr Fantastic, Ms Marvel and the Super-Skrull overthrew the prison and freed the slaves. They then spent a year local time refitting UX-73 as a mobile battle platform, powered by Praxagora’s fusion core and manned by the freed prisoners, while also building a viral missile to destroy the Harvester.

Now in the positive matter universe, the UX-73 platform is a major resource in the battle against the Annihilation Wave. It has faster-than-light drives, a powerful force field and multiple energy cannons, and is crewed by a variety of sentients with assorted skills and abilities.