Hollow

The Hollow is the 4th chamber of The Gungeon. It resembles a large, icy ossuary and introduces various undead enemies. In addition, it occasionally features natural floor ice that proves a slippery surface for players but not enemies.

If you enter the The Hollow with a Master Round in your inventory, a special room with the Sell Creep will always spawn. If the player gives the Sell Creep a Master Round and two other items, the player will gain access to the R&G Dept.

Trivia

 * According to the Hollowpoint Ammonomicon Entry, the Hollow is where slain Bullet Kin are sent to, either to rest or be used in some form of unholy Necromancy/Ammomancy. Since skeletal remains of non-bullet based creatures, mostly humanoid, are found in the Hollow, it is possible this does not apply only to Bullet Kin.