A softlock is an unintended situation in a game where it is not possible to proceed towards the end. Most games that do not auto-save can avoid this by reloading the last save. In Dark Souls, there exists one instance of a possible softlock, the Priscilla Moan Glitch.
In the Crapplemage route there is a small chance that the player can cause the game to be stuck in the Painted World of Ariamis. This has a chance of happening when the player deals enough damage to Priscilla (generally by using Dark Bead while in Red Tearstone Ring (RTSR) range) to defeat her instantly. It causes Priscilla's dialogue to loop which stops the player from leaving the Painted World of Ariamis. Any attempt to leave (Darksign, Homeward Bone or death) will spawn the player back at the start of the area and Priscilla will still be looping her dialogue when the player returns to the boss zone. This effectively stops a run since the player cannot leave and finish the game anymore. There is no known way to fix the glitch once it occurred outside of external programs or save file manipulation, which is completely forbidden in speedrun attempts.