Debunking the ubiquitously incorrect information on this topic, to help those struggling, by using results of careful in-game testing in Core Keeper Version 0.3.5-05e7.
Requirements for NPC to Teleport into a Room
- (1) Fully enclosed by walls of any type.
- (2) Summoning item – “Slime Oil” or “Mysterious Idol” (from defeating bosses).
- (3) Bed – crafted (not “Caveling Bed”).
- (4) 1 empty tile of space – that includes no torches.
- (5) Minimum floor space of 4 tiles (e.g. 2×2), as dictated by the above requirements.
- (6) Max size 63 tiles (i.e. 8×8 – 1).
- (7) Their existing bed is picked up (if there is one). Picking up only the summoning item or breaking a wall/door is not sufficient.
- (8) Player moves away so that new room and NPC themself are entirely off the mini-map for some amount of time. 10 seconds seemed to do it for me, but it may be inconsistent, so try going further for longer if stuck.
- (a) Door.
- (b) Light source (NPCs have their own).
- (c) Specific floor covering or wall type (natural dirt will suffice).
- (d) Separate room – NPC will share, provided there’s enough beds.
- (e) Clean bed – it *can* be player’s respawn point.
- (f) Entirely solid floor – holes are fine, provided enough good floor space.
- (g) Safety – room may contain enemies and their spawn surfaces (e.g. slime).
Advanced Tips and Tricks
- (i) Placing the summoning item in the doorway can let you past while blocking them in. They seem to have a bigger collision box. But it must be a side door with a clear tile above the Slime Oil/Mysterius Idol. Other furniture can be used to force the NPC’s position in bigger rooms.
- (ii) Spoiler for late game: NPCs can be summoned to the wilderness, then (given their immortality) used as a punching bag to hit a crit and trigger your Soul of Azeos Thunder Beam, which is able to break through Mould Dungeon walls.