Septim was the guardian of Werthrent's heart, which held one of the Dimensional Keys.
Background[]
Septim was an old, human looking man, who lived inside the body of Werthrent.
After being devoured by Werthrent, the Crimson Light's crew realized that Werthrent's heart held a Dimensional Key. Gary Goodspeed and Little Cato tried to take the key, but Septim stopped them. Upon asking the crew who they where, he recognized Ash Graven as a child of Werthrent (due to her being chosen to be sacrificed to him when she was young), and deemed the others "unworthy".
When Gary demanded to get the key, Septim challenged him to a series of three complex riddles. While the riddles left Gary baffled, KVN easily answered the first two. Septim then cheated and made the third riddle a bunch of nonsense that even KVN could not answer, but KVN realized this and angrily threw his Forever Ball at Septim. The ball hit him repeatedly, and with each hit crushed Septim's old and brittle bones, brutally injuring him.
As Werthrent's dimension was destroyed after Little Cato took the key, Septim perished along with everyone trapped there.