Darksiders III is an unconfirmed titled in the Darksiders Series.
There has been no official announcements regarding the development of Darksiders III, however, some of the developers behind Darksiders II have commented about the possibility of a new game in the series. Darksiders II lead designer Haydn Dalton in particular stated that if Darksiders II sells very well, the team could in fact go on to do another game in the sequel.
In particular Dalton stated that a third game might have a slightly reduced scope, but something that is much more polished. Instead of having a massive overworld like Darksiders II has, it could have a much smaller and dense overworld. Where there is more interaction to the environments and a lot more to do. The importance isn't necessarily on the sheer size of the world, but how much there is to do.
Creative Director of Darksiders II Jeremy Greiner commented on the possibility of Darksiders III. "So, will there be a Darksiders III, Darksiders IV... I hear that question all the time. Ultimately in the end, the studio started the Darksiders franchise to tell the story off all Four Horsemen of the Apocalypse. So we'd like nothing more than to tell you the stories of Strife and Fury, and then maybe one day get all the horsemen into one game together and have some fun with that."