Hundun will rise up on a platform of spikes in its lair, forcing the player upwards through Sunken City-style world generation to the arena portion of the level.
- Hundun walks towards the direction of the player. Its legs and feathered portions damage the player for 1 HP if touched.
- As Hundun ascends, 2 heads will slowly burst out from its shell.
- After rising up about 3 rooms (18 tiles), the head of a bird will burst out, with 25 HP.
- After another 2 rooms (12 tiles), the head of a snake will burst out, with 30 HP.
- Both heads spit fireballs at the player at regular intervals.
- Fireballs deal 2 fire damage, igniting and stunning what they hit. They dissipate on contact with water.
- Attacking a head just before they spit a fireball cancels the attack.
- If one head is attacked, the other will always prepare a fireball, if they are not killed or still in its shell.
- Once Hundun's platform stops ascending near the top of the level, it sprouts wings, which it uses to fly in the direction of the player.
If both heads are killed, Hundun will collapse to the ground with its eye open, staring at the player. Hundun no longer deals contact damage in this state. The door to at the top of the level also opens, allowing the player to see the second ending of the game.
- Hundun is a figure in Chinese mythology/cosmogony.
- In version 1.15.0 and prior, there was a glitch allowing the player to shoot Hundun's closed eye with the Arrow of Light, granting access to the Cosmic Ocean without defeating Hundun. This was fixed soon after its discovery.
- According to Spelunky creator Derek Yu, a Lovecraftian-style boss fight for the Sunken City was originally considered during development, although Hundun was ultimately decided on. The centerpiece seen in the main menu door was created while a Lovecraftian-inspired boss was still in mind.[source][imgur mirror]