The player starts in Room 0 with the Axe already in hand. The gameplay is exactly the same as the main game, although the mansion obviously continues endlessly instead of ending after Room 1000.
There are save points every fifty rooms until Room 200, where they occur every 100 rooms instead. After Room 1000, save points are available every 200 rooms. Dying will lead to the deletion of the player's save file, forcing them to start anew. Quitting the game is also disabled during a chase.
- Specimen 1 - A lot of new designs including a boot, doughnut and candy corn as well as new sound effect clips.
- Specimen 2 - New death screen.
- Specimen 3 - No notable changes.
- Specimen 4 - A second variant occasionally appears, having different gameplay.
- Specimen 5 - Death screen was added, increased walking speed, new hallucinations.
- Specimen 6 - No notable changes.
- Specimen 8 - New scene on its death screen.
- Specimen 9 - Only appears if the player is idle for too long.
- Specimen 10 - No notable changes.
- Specimen 11 - New hallucinations added.
- Specimen 12 - Can either chase the player in docile or active mode.
- Monster 3 - New chase added. Please see page for more details.
- Monster 4 - New chase added. Please see page for more details.
- Monster 5 - New chase added. Please see page for more details.
- Monster 6 - New chase added. Please see page for more details.
- Unknown Specimen 1 / "White Face" - The floating 8-bit White Face from the popular indie horror game IMSCARED, making a surprise appearance. Makes loud noise as it gets closer to the player. Screen flashes red at random intervals.
- Unknown Specimen 2 / "Otto the Otter" - An animatronic Otter with a deep laugh. Moves very slowly and does minimal damage.
- Unknown Specimen 3 / "Spooper" in the files - Child ghost/parasitic mutant. Slowly drains player's health in every room. Has an ability that temporarily freezes the player and makes them vomit, which takes away a portion of health.
- Unknown Specimen 4 / "Tirsiak" in the files - Forest warden girl. Shadow creatures block the path, player must kill them to progress. Entire rooms become bright blue. Player can see the shadow of the girl from far away.
- Unknown Specimen 5 / "Lisa" in the files - Before she appears, rooms gradually turn red and popouts with short, ominous notes appear. During the chase, Lisa behaves similarly to Specimen 6 and Monster 4, only she teleports at a faster speed.
- Specimen 7, Specimen 13, Monster 1, Monster 2 and Monster 7 are the only enemies that don't appear in the Endless Mode.
- Notably, the Endless Mode lacks special Rooms such as the Foggy Maze or Minigames Room.
- However, the Endless Mode contains a lot of new rooms that don't appear in the main game.
- If the player dies in the Endless Mode, the game over screen will say "Your soul wiggles in agony", which is different from the original game's and Karamari Hospital's "Your soul wanders in an endless maze" game over screen.
Endless Mode returns in Spooky's HD as a free patch on October 20th with differences from its original counterpart.