Darker Dungeons is a pen-and-paper role-playing game. It is a sister game to Dark Dungeons.
Like Dark Dungeons, it is based on a specific out-of-print edition of a specific game which cannot be named for legal reasons. However, unlike Dark Dungeons it doesn't try to faithfully emulate that game. Instead, it tries to bring it up to date by replacing many of the disparate game mechanics with a unified system (although it is careful to keep the probabilities of success/failure of actions the same to within +/-5%, so the feel of the game is very similar to Dark Dungeons).
Additionally, Darker Dungeons takes advantage of the fact that it does not need to stick to its source material as closely by including other changes, including:
Ability Scores and hit points no longer being randomly generated.
Changes to classes (the human Mystic has been replaced by the demi-human Lupine, the Mountebank has been added, and the Magic-User and Elf have been swapped).
Removal of Alignment.
The ability for clerics and magic-users (and elves) to specialise in one or more types of spell at the cost of being less good with other types.
Shields no longer giving a flat bonus to armour class, but instead characters spending Proficiency slots on them to get their bonuses, just like weapons.
The removal of Shield Weapons (Knife Shield, Horn Shield, etc.), and the addition of Flails and Morning Stars.