When Apophis had ordered him executed, Teal'c managed to spare Del'mor's life. Later when Teal'c rebelled Del'mor believed it to be a sign and sided with Bra'tac when the Jaffa rebellion was in its infancy. He even burned the serpent tattoo from Rak'nor's forehead, telling his son that they were now free. But Del'mor and his entire family were slain, likely when Apophis returned with Sokar's forces and brought havoc and death to Chulak. Rak'nor, bowing to Apophis's power, managed to survive.
Rak'nor would later believe his father to be a fool. With Apophis's rise from the grave and defeat of Sokar, he was convinced there was no way for the serpent lord to not be a god. But when Teal'c himself renounced the Goa'uld time and time again before Lord Terok and a vocuum, Rak'nor was eventually swayed to his father's beliefs.
The Serpent's Venom - Rak'nor reminds Teal'c of his father Del'mor, a Jaffa whose life he saved when Apophis ordered him dead.