Ipslore the Red
Death therefore has to wait some years to collect the soul of Ipslore, who in the meantime is instructing his son via the staff and can be seen to be the real Fifth Elephant behind the sourcery - unable to perform the magic himself he is nevertheless the arbiter of it through Coin.
Death allows Ipslore to retain his unclaimed soul within the staff, as breaking it would also bring about the boy's untimely death. The condition is that Coin has to throw away the staff of his own free will: only then would the bond be broken and the soul of Ipslore be claimable by Death.
While this agreed to be a possibility of molecular size, it does not account for Rincewind, who is the agency by which the staff is rejected: Coin disobeys an order from his father to kill Rincewind and throws the staff away in disgust.
Ipslore seemed to be a powerful wizard but even by wizard standards on the Disc was considered slightly unbalanced. Himself the eighth son of an eighth son, he is ambitious and believes in what he does; he believes in sourcery and clearly loves women as he then fathered eight sons, each as powerful from the cradle as any wizard on the Disc, the eighth being a wizard squared: the sourcerer. Although he eludes Death at first, when Death comes to claim him Ipslore is highly emotional and driven.
His staff is made of Octiron which is rare and he has great pride in this as wizards' staffs are made of wood and he claims that he was laughed at when he said he would make a staff of metal. However he ends up with a most effective staff in which he places his essence and which is then carried by his son.
At the denouement, when the 'fluence over Coin is broken and the staff discarded, Rincewind heard Death say, triumphantly, Ipslore the Red, I have you now!