John Romero, one of the developers behind the seminal computer game Doom, has released his first new level for the title in 21 years.
He is offering the new material for free via the file-sharing site Dropbox.
Mr Romero told the BBC he had made it as a "warm up" for a new project.
Doom was released in 1993 and had 10 million players by 1995. It has retained a cult popularity, and still has a dedicated community of players who also create their own content.
The source code - the computer program that powers the game, written in a human-readable computer language - has been available for some time.
As a result, Mr Romero estimates there are about 30 alternative versions of the entire game, and more than 100,000 add-on levels created by Doom fans.
"Every day people are still making new levels for it," he said.
His new level, Tech Gone Bad, is a boss level that offers an alternative climactic fight.
It ends in a massive complex with a huge computer room that links to different portals and "hell cracks" in the floor, he said.
The original final level was made by the developer Sandy Petersen and not Mr Romero.
The new version is mostly based outdoors.
Mr Romero said that distinguished it from the original game, which was largely based on indoor play, with the exception of Mount Erebus in level six (E3M6 in the Doom register).
"Doom mapping is pretty easy to do compared with 3D engines," Mr Romero said of his own creation.
"But I did worry... the community that's been playing it for 20 years would tear [the new level] apart."
However the reaction from players has been positive, he added.
"It's been out for four days and it's exploded. It's crazy that a game that's 23 years old has so many people still super-excited about it."
He added he had no idea Doom would become such a perennial favourite back in the early 1990s.
"With Doom, it was mouse and keyboard-controlled on the PC when it was released, and you can still play it that way," he said.
"People remember how much fun they had a long time ago and I hope they will have that much fun again."