Terror Returns is a mod for registered Wolfenstein, made by Thomas Hubert. It was released on March 24, 2000.
The mod features 10 new levels, played on Episode 1. A number of graphics have been changed, with some being unique to the mod.
As was mentioned by Brian Lowe, Hubert's mapping skills had improved significantly compared to earlier efforts.
Being a map set, it is playable in SDL.