Grimdark

Grimdark is a term used by science fiction/Fantasy fans to describe fictional worlds with excessively bleak settings. The term originates from Warhammer 40,000's slogan of "In the grim darkness of the far future, there is only war".

The term also is also used to re-imagined takes on frachises endevoring to become grimmer/more mature. An example of which is nBSG which is occasionally defined as Grimdark.