A Sub-Boss, also referred to as a Room Guardian, is an enemy you encounter along your way. They are usually fought within a closed room (the entrance/exits won't open until you beat them). The usually aren't as powerful as guardians and unlike a guardian battle, you don't need an Ankh Jewel to unlock them.
List of Room Guardians
- Algol: found on the Surface at B-3.
- Ba: found in the Confusion Gate at D-1.
- Beelzebub: found in the Shrine of the Mother and the True Shrine of the Mother at D-5. He will only attack you in the True Shrine of the Mother.
- Big Man: found in the Chamber of Birth at F-5 after obtaining the Flywheel and performing the "Dance of Life".
- Buer: found in the Temple of the Sun at A-2.
- Chi You(Shiyū/Shu): found in the Confusion Gate at D-4.
- Gozu & Mezu: found in the Chamber of Extinction at E-1.
- Hekatonkheir (Centimani): found in the Chamber of Extinction at C-4, and in the Temple of Moonlight at B-5.
- Kamaitachi: found in the Graveyard of the Giants at E-3.
- Nu Wa: found in the Tower of Ruin at E-1.
- Nuckelavee (Jackrabbit): found in the Spring of the Sky at E-3.
- Pazuzu: found in the Inferno Cavern at B-3.
- Peryton: found in the Twin Labyrinths at B-3.
- Thunderbird: found in the Tower of Ruin at D-4.
- Zu: found in the Twin Labyrinths at I-3.
Dimensional Corridor Sub-Bosses
- Girtablilu: found at C-5
- Raham: found at E-7
- Urudimmu: found at C-4
- Storm: found at E-3
- Bashumu: found at E-5
- Creel: found at B-7
- Ox: found at B-5
- Mushumahhar: found at C-3
- Dragon: found at D-2
- Mushussu ("Shirrush"): found at A-4
- Ugallu: found at D-4