Pixel unDungeon is a small patch on top of Pixel Dungeon which adds an undo button to the in-game menu (available even after death). Pressing undo will return you 10 turns back in time.
If after dying you choose to exit game or return to menu you the game will be over permanently.

The original Pixel Dungeon is a traditional roguelike* game with pixel-art graphics and simple interface:

Many people consider the original Pixel Dungeon game very difficult and luck-based. Anyway, you will die often. The undo function may help to reduce frustration!