Each time you enter any level the first time in the given playthrough and you have died here in any previous game, there is a small chance that instead on generating a new level, a bone file will be used. That way you can recover [[items]] you had on your old adventurer, but all other features on that level will be saved as well - the [[Monsters|monster]] that had killed you, all physical alternations alterations to the dungeon made by you etc.