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 appear with ominous notes attached to them. During the chase, Lisa behaves similarly to Specimen 6 and Monster 4, only she can teleport at a much 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 returned in Spooky's HD as a free patch on October 20th, 2017, with differences from its original counterpart.
In contrast to the original's Endless Mode, where you started in a random room, you will now begin outside the mansion, with a boarded-up door in the middle of the area. You have a choice of which weapon you will take with you; the Axe is lodged into the frame of the door, and the Sword is on a operating table found at the top-right of the starting area (only if the player has cleared Karamari Hospital at least once). The decision appears to be purely aesthetic, as both weapons function exactly the same, and both affect the same specimens.
Exiting the game during a chase via closing the window will now delete the player's save file as if they had died.
Also unlike the original, each Specimen's scripted room from their original appearances (i.e. Story Mode and Karamari Hospital) will also appear. In each floor of 100 rooms, one of these rooms will randomly appear. From that point on, the specimen in question will be available to chase the player, much like the previous Endless Mode. Specimen 2 is the sole exception to this rule, as it is in the "Rechase Pool" (explained below) by default, though its starting room can be encountered rarely. New starting rooms have also been created for the Endless Mode-exclusive "Unknown Specimens" White Face and Spooper still do not have proper starting rooms).
It is now possible to encounter more than one specimen at once, significantly increasing the difficulty. This includes getting chased by 2 of the same Specimen or getting chased by multiple Specimens. The player can encounter a maximum of 5 Specimen simultaneously. The way this works is assumed to be as such:
- When the player reaches a randomly predetermined starting room for a specimen, that specimen's chase begins.
- After the chase begins, the specimen is added to a "Rechase pool."
- The specimen is still added to the pool if the player has disabled specimens in the Options menu.
- At the start of each room, the game checks to see if the player can encounter a specimen.
- This check occurs even if the player is already being chased by another specimen.
- This check automatically fails if the player is inside of a "Safe room" (Elevator, Foggy Maze, Brain Room, LP Room, or the Minigames Room), or if they are already being chased by the maximum of 5 Specimens.
- If so, the game then randomly selects a specimen from the "Rechase pool" to spawn in that room chasing the player.
- Each specimen has an equal chance to spawn.
- This can include another copy of a specimen already chasing the player; this earns the player the "Seeing Double" Achievement.
- Note that Specimen 2 is in this "Rechase Pool" by default.
- Specimen 7, Specimen 9, and Specimen 13 are never added to the "Rechase Pool."
- The game then repeats Steps 3 and 4 for each room, until Step 1 is needed, or the player dies.
- Specimen 1 - No notable changes.
- Specimen 2 - Due to the new mechanics of endless mode, Specimen 2 will appear much more often at the beginning of the game, due to being the only "default" specimen available. It does have a starting room, which is identical to its starting room from Story Mode (Room 60), but encountering it is very rare, due to the new mechanics. It can be attacked with the Sword and or Axe.
- Specimen 3 - Its starting room is identical to its Story Mode counterpart (Room 120). It can be attacked with the Sword or Axe.
- Specimen 4 - The first form's starting room is identical to its Story Mode counterpart (Room 160). Her second form now has its own starting room: a short school-like corridor with a wall of static. Entering the static will dismiss it and Specimen 4 will appear. The mechanics of her second form's abilities have been changed. Instead of a constant static overlay over the player's vision, it will now take the form of a wall of static appearing in the room, most often near the exit. This wall will come and go similarly to her previous ability. Her third form can be seen but is rarer than the original endless mode, and appears to go slightly faster than the second form.
- Specimen 5 - Its starting room is identical to its Story Mode counterpart (Room 210).
- Specimen 6 - Its starting room is identical to its Story Mode counterpart (Room 310). He can be attacked with the Sword or Axe.
- Specimen 7 - Its starting room is identical to its Story Mode counterpart (Room 410) which has changed a lot from the original, although it will not give chase more than once.
- Specimen 8 - Its starting room is identical to its Story Mode counterpart (Room 550). However, you do not need the key to open the middle door before encountering Specimen 8, as it is already unlocked. W.A.M. can still be encountered as normal. The Axe still appears lodged into a tree, despite the fact that you physically cannot reach this point without having a weapon already.
- Specimen 9 - Its endless corridor can now appear at random. It will also appear if the player remains idle for more than a minute without pausing, and will kill them on contact.
- Specimen 10 - Its starting room is identical to its Story Mode counterpart (Room 610). It takes less distance and less time for it to transform into its deadly second form.
- Specimen 11 - Its starting room is identical to its Story Mode counterpart (Room 710). It can be attacked with the Sword or Axe
- Specimen 12 - Its starting room is identical to its Story Mode counterpart (Room 810). This includes the hiding scenes.
- Specimen 13 - Its starting room is identical to its Story Mode counterpart (Room 910). However, it will only give chase once.
- Monster 2 - It can now randomly appear. Its starting room is a circular room filled with body bags, similar to its original room from Karamari Hospital. The player no longer has infinite stamina during the chase. It will speed up the longer the player is in the room; this can result in it actually being faster than the player, even when running. When it hits you, it will teleport back to the entrance of the current room.
- Monster 3 - It can be attacked with the Sword or Axe which will teleport it to an area in the room where the player is not facing. Its starting room is a small, 2-way rusted catwalk. It will appear at the center of it and will proceed to chase down the player.
- Monster 4 - It can now be attacked using the Sword or Axe. Attacking it will make it disappear for the rest of the room, much like Specimen 6. Hitting it enough times will end its chase, much like in Karamari Hospital. Its starting room is identical to its counterpart in Karamari Hospital. However, the hallway normally leading to Monster 6 is blocked with crates.
- Monster 5 - It can be attacked with the Sword or Axe which will knock it back to the entrance of the current room. It's starting room is a rusted maze similar to the Isolation Room from Karamari Hospital. It now moves much faster, but can no longer walk through walls or teleport. It is no longer guaranteed to transform the room.
- Monster 6 - Chase mechanic has been changed. It will occasionally appear infront of the exit door as the player approaches it and damage them before going away. Its starting room is identical to its counterpart in Karamari Hospital, with the difference that the hallway is not nearly as long. Also, the door normally leading out is locked, forcing the player to walk down the hallway. It is possible to avoid taking damage by her, this can be done by getting close enough to the door to make her appear then she will stay there for a few seconds then she will disappear from the current room.
- Unknown Specimen 1 - When White Face first appears, the sound effect will play and the screen will be covered in a pixelated overlay, but it will not change the sprites of the room yet. White Face also appears harmlessly in front of the entrance door and will stay there until the player leaves the room; this is considered its starting room. Further rooms will proceed as normal, complete with the old-school styled doors and rooms. After a few rooms, White Face will give chase. White Face itself has been considerably nerfed in speed and teleports away less often, being able to be directly observed for a few seconds before disappearing. It will also occasionally appear from the exit door and follow the player for a few seconds, similar to Monster 6. Its second chase (triggered when the player types "Open" into the box after having their health depleted) is now much easier.
- Unknown Specimen 2 - Now possesses it's own starting room; a pizzeria-like restaurant, very reminiscent of the Five Nights at Freddy's franchise. At first, Otto will be standing harmlessly on a podium with a slice of pizza next to him. Consuming the pizza will display the text "DELICIOUS", and will awaken Otto, beginning the chase with a short jingle. Otto is now larger and faster, making him a legitimate, albeit minor threat.
- Unknown Specimen 3 - This specimen will now appear randomly at a door in it's "bed-sheet ghost" form, which doesn't have many changes made to it other than being able to take one more hit before turning into it's "parasite" form. It now has the ability to teleport in front of other doorways to block the player's path to the next room, rendering it impossible to delay its chase, as you could in the original. The parasite version of Spooper is much smaller and slightly slower than its original counter part, and is seen less often. The first time it appears is considered it's starting room. The extremely bloody bed-sheet ghost form of Spooper can appear even when the parasite version is present; this is thought to be a part of the hallucinations. Touching these hallucinations may trigger the vomiting effect of the parasite in the next room.
- Unknown Specimen 4 - Now has it's own starting room, reminiscent of a snowy tundra forest that has a little cabin on its left and a few big trees on its right. The chase will start about halfway into the room. Tirsiak's shadow animals are now far larger and take longer to kill due to the new Axe and Sword mechanic, making her a far greater threat. She will also no longer get knocked back when she attacks; this is replaced with a simple, roughly 2 second attack delay.
- Unknown Specimen 5 - Possesses it's own starting room, which resembles a house-like interior with several bookcases. The room will begin to loop, and will change with each loop. A phone will appear and constantly ring, the walls will become deteriorated, and the screen will get redder in tint. This then exits the starting room and applies the red tint to regular rooms. At this point, Specimen 1 will start to appear, with the threatening notes attached to them. After enough loops, Lisa will appear and give chase.
- The developers have also confirmed that they will be adding more specimens for Endless Mode in the future.
- With the addition of Specimen 7, Specimen 13 and Monster 2, Monster 1 and Monster 7 are the only enemies that don't appear in Endless Mode.
- It is now possible to encounter rooms not originally scripted for the original endless mode, including Specimen 9's endless hallway, the brain room, the Minigames room, the foggy maze room, and Howard's room, although the CAT-DOS room still cannot appear.
- If the player presses ESC while in a chase, the option to quit has been disabled and the text has been changed to 'No Escape'