The seventh and youngest son of the 81st Lord of Stormhold, Septimus is a bit of a puzzle: "Septimus, his black beard still coming in, tall and crowlike, looked blank, as he always looked blank" (3.12). There's darkness inside him, though, and he's a murderous type of fellow, having killed two of his siblings already (Quintus and Sextus); he manages to off Tertius as well.
We learn that "by temperament, Septimus was one of nature's poisoners. Blades and blows and booby traps were well enough in their way, but a vial of clear liquid, any trace of taste or odor gone when it was admixtured with food, that was Septimus's métier" (9.5). Well, it's good to know that he has a hobby.
Fate helps Septimus out by killing Primus… but Septimus still has to avenge his brother's death before he can seek the topaz that will make him Lord of Stormhold. Ugh. He is not thrilled about this, telling the corpse, "'now I must revenge your sad carcass, and all for the honor of our blood and the Stormhold'" (8.9). Tell us how you really feel, why don't you?
Septimus decides to try to take revenge on the old woman who killed Primus by setting her hut on fire and waiting outside the only door with a club. She gets the jump on him, though, by having a poisonous snake bite him with a lethal dose, which is somewhat ironic given his own fondness for poisons.
The poison kills him, and even if it hadn't, would he really have had a chance against one of the Lilim? Sorry, Septimus, you're pretty crafty, but we think you're out of your league in this fight. His ghost joins those of his brothers, and now that they know that none of them will attain the throne, they vanish from the earth.