Just stumbled upon an apparently very obscure DOS first-person action/adventure game from 2000 made by a German programmer Florian Born that is called Desperabis
. According to the game's home page
(archived copy), the full game was released as freeware on December 17, 2000. I have not yet found any information regarding its distribution prior to that date (i.e. whether it was free or sold). Info on the game is rather scarce, but it does feature some decent graphics, judging by screenshots.
The full version can be downloaded here: DESPERA2.EXE