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 is also used in reference to re-imagined takes on franchises endeavoring to become grimmer or more mature, even if in said transition they don't come anywere close to the level of bleakness of things such as Warhammer 40,000 and similar. An example is nBSG which is often defined as Grimdark due to the significantly darker tone when compared to the original series.