Alaster is highborn, gifted, and raised to expect that the world would meet him at his level. It did not. His whole life is the quiet arithmetic of a debt he believes is owed to him and has never been paid.
The promise the dark makes
The Void's genius is that it does not recruit the desperate half so well as the entitled. Where others hear an ending, Alaster hears an offer: strength now, answers now, a settling of accounts. It answers faster than the light ever did, and it never once tells him no.
Under Vorst's hand
He falls into the cults not as a fanatic but as a talented man who is certain he is using them. Vorst, the Cult-Master, lets him keep thinking so. Alaster is a knife that believes it is the hand, and by the time he understands the difference it may be far too late to matter.
The ordinary fall
His is the most frightening arc in the saga because it is the most ordinary. Not madness, not grief, just ambition given a door. He is the study in how good material goes to the Void, and the whole question of his story is whether any of it can come back.