Terciel was the father of Sabriel and Lirael, by different mothers, and was the Abhorsen before Sabriel. He fathered Lirael with a Clayr named Arielle, approximately eighteen years after the death of Sabriel's mother Elinor in childbirth.
The circumstances surrounding Sabriel's conception are largely unknown, except that her mother was very dear to Terciel. Elinor died shortly after giving birth to Sabriel in a Traveler camp, and Sabriel followed her into Death. Terciel arrived shortly after, identified himself simply as "Abhorsen," and pursued Sabriel's soul into death. He caught up to her before she reached the first gate, only to find that the Greater Dead being known as Kerrigor had found her first and was holding her hostage. Terciel was able to secure Sabriel and send Kerrigor back into Death, and returned Sabriel to her body without doing any lasting damage to her. He then secured the service of the Traveler's Midwife, as well as their Charter-Mage, to take care of Sabriel during his travels around the Kingdom.
During Sabriel's time in Ancelstierre, in Wyverly College Terciel visited her every full moon via Sending. Despite their relatively infrequent communication, they had a very close relationship--although Terciel seems to have withheld his real name from her. Unfortunately, shortly after Sabriel turned eighteen, Kerrigor returned and trapped Terciel inside Death. He managed to deliver the Abhorsen's Bells to her via a Dead creature he managed to bind to his will.
Sabriel's original goal was to free her father from Death, but when she finally made her way to the reservoir below Belisaere, the Old Kingdom's capital, she found that Terciel had been gone from his body too long to survive outside of Death for long. He was able to escape Death with his daughter, briefly, but chose to stay behind and use what little time he had left to hold Kerrigor off, giving Sabriel and Touchstone a chance to escape.
Terciel died battling Kerrigor by the broken Charter Stones in the Palace Reservoir, whilst wielding the bell Astarael. The Bell was returned to Sabriel before her final confrontation with Kerrigor, where she was able to bind him with Ranna, in much the same way as Mogget was bound. Kerrigor has yet to awaken from the slumber that his binding forced upon him, and, according to Mogget and the Disreputable Dog, never will.