Assassinate Hitler is a mod for registered Wolfenstein, released in 1998. It was made by Gary Ragland.
It was one of the earliest mods to modify the source code. This was done by Chris Chokan, who also released his own mod, Chokage, in August 1998. Assassinate Hitler was for a while available as an add-on to the Chokage EXE, titled Hitler20.
The mod follows a seamless level flow, among the first if not the first to do so. 20 levels are featured.
The storyline as featured in the Read Me file:
You are to Assassinate Hitler, dealing one final crippling blow to the Third Reich. Adolf Hitler, realizing that the end
is near, has escaped to his TITANIC bunker in the German countryside. Allied intelligence has just now determined its location, and you must go there to perform your mission. The bunker itself is located underground and is bomb-proof, but you wont be able to access it immediately. First, you must blast your way through an ancient German castle, which sets atop the entrance to the bunker itself. (This segment is represented by the first 10 levels of the game). The last 10 levels represent the bunker itself, which is a little more hectic than the castle levels. Level 20 is where you'll finally confront Hitler...
Level 8 features a particularly difficult secret area section, obviously inspired by original Wolfenstein's E2M8.
For a long time, the downloadable version of the mod featured a different starting point in level 10, being close to the exit. This was later corrected in the SDL port.