Umbra
Umbra are shadowy creatures found in Sheol, usually in larger numbers. They will move around in quick, zig-zagging dashes and switch between chasing after Isaac, and moving about randomly.
If another Umbra is nearby, they will move together and merge into a Blistered Umbra and combine their remaining HP.
Blistered Umbra
Blistered Umbra are 2 Umbra who have fused together, in a diagonal orientation. They behave similarly to Umbra but have much slower and smaller dashes. If Isaac draws near or lines up with them diagonally along the line of their orientation, they will quickly split up, launching Umbras in each direction. Each Umbra will have half the remaining HP of the Blistered Umbra. If a Blistered Umbra is killed, both Umbra will die.
The direction which the Umbra merged will influence its diagonal appearance.
Eclipse
Eclipse are stationary, invulnerable portals that will periodically spawn an Umbra. Eclipses will disappear once all other enemies, not including Umbra and Blistered Umbra, are killed. Tears will pass over an Eclipse and they will not deal contact damage, but will block Isaac's movement.
Eclipses have a limit for how many Umbra they can spawn that depends on the room layout, ranging from 6 to 12 Umbra in certain room layouts (Blistered Umbra are considered 2 Umbra).