Shuma-Gorath is from a time before the current time. Another universe with different gods, different worlds. When that time/universe collapsed, Shuma-Gorath was able to escape and not be trapped in the prison plane of Gejora, where something was seemingly trapped there.

Shuma-Gorath pulled a half-elf named Ulbane from this previous time and dropped him across the time spectrum. Watching the Ascension of the Everqueens, helping Jorun kill the Quintessence, and finally putting him in Oakenshield. Ulbane was the key to bringing Shuma-Gorath fully to this plane.

Once the Priest of Shuma-Gorath, who was residing in the Feywild, was killed which allowed the trapped Archfey, the Erlking to be released, Ulbane took over as the Aspect of Shuma-Gorath on this plane. This allowed Shuma-Gorath to start advancing its plans as Queen Mab had to deal with the Erlking rather than keeping Shuma-Gorath in its holding pattern.

Once the ritual was complete (though it was largely foiled), Ulbane became the Scion of Shuma-Gorath. A physical aspect that had encroached on this world to start the process of fully bringing this creature to the plane.

But, the ritual was stopped, the Scion of Shuma-Gorath was killed, and no Aspect of Shuma-Gorath currently remains.