In Cloisterwood, you can come across an Undead trader called Eithne.
Speak with her. You learn that she wants to free herself from a life of Undead. The woman asks you to bring her a book that contains knowledge on Corpse Explosion ability.
There are no books that can teach this ability. You must create it by yourself. Combine any of the books from Necromancer and Pyrokinetic schools. If you don't have any at the moment, you can buy them: a book from Necromancer school can be bought from Eitine and books from Pyrokinetic school are sold by Ovis in Driftwood.
Then, you have to bring the book to Eitine. She wants to buy it from you. You can give it for free or exchange it for something from Eitine's offerings. Regardless of your choice, the trader will leave you in order to end her life. If you want to complete this quest, you must go to Stonegarden and visit Eitine's grave.