More details on merchant inventories, etc, in this wiki page[corekeeper.atma.gg].
0.5.1.0 Christmas Seasonal Event:
• Merchants will now teleport into suitable rooms immediately, even with player close by, on screen.
• Maximum room size increased from 63 to 64 tiles.
• Added new Seasonal Merchant NPC summoned with a "Seasonal Calendar" crafted at Carpenter's Workbench.
0.4.0 Sunken Sea:
• "Merchants no longer drop all their inventory on death."
• "Merchants will follow the player when the player holds [their summoning item]."

(1) Fully enclosed by walls of any type (corner cracks are fine).
(2) Summoning item - "Slime Oil", "Mysterious Idol" or "Bucket of Chum" (from defeating bosses) or "Seasonal Calendar" (crafted at Carpenter's bench).
(3) Bed - crafted (not "Caveling Bed").
(4) 1 empty tile of floor space - without torches, conveyors* or 2x2 "Stone Tiles" or 3x3 "Core Relief" tiles (from Sea dungeons).
(5) Minimum floor space of 4 tiles (e.g. 2x2), as dictated by the above requirements.
(6) Max size 64 tiles (e.g. 8x8).
(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) They will teleport in while room is on-screen and player is in the new room. But may linger if player is close to their current position.
(9) If they've not moved in restart your game (e.g. exit to main menu and reload) or restart you dedicated server.
(10) If still stuck, try going over to where your NPC currently is, if far away, to wake them up. Over short distances, they can be led by holding one of their summons in your hand.

(a) Door.
(b) Light source (NPCs have their own).
(c) Specific floor covering or wall type (natural dirt will suffice, bridges too).
(d) Separate room - NPCs 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 there's 1 tile empty floor space.
(g) Safety - room may contain enemies and their spawn surfaces (e.g. slime).

(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 trick: NPCs can be used as a punching bag to hit a crit and trigger your Soul of Azeos Thunder Beam (given their virtual indestructibility). Use this to either break Mould Dungeon walls, by summoning them to a box house right outside where you want to enter.
Credit: u/Vicidomini.

Or to make a line of permanent illumination (for decorative uses). Canonically, the beams break the ceiling, letting in sunlight. This can be shut out again by placing block in those tiles.


Edit 2022-06-16: Removed NPC farming sections that are defunct as of 0.4.0 "Sunken Sea".

Thanks to Discord user nineforty for additional testing in 0.3.13.

Fellow creatives: if this material inspires some of your content, I'd appreciate a quick shout-out and link back to this guide. 👍

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