Mortod was the son of Kolb and Delka.
Delka and Mortod were killed in 2381 on Deneva during the Borg Invasion. Delka brought Mortod with her on the trip to Deneva, thinking he would enjoy a trip to another world. The loss of his wife and child devastated Kolb, and he planned for years to join Project Nexus for a chance to see them again.
Kolb and Jean-Luc Picard met an illusion of Delka in the Nexus in 2384, and Kolb was unwilling to leave the experience behind to return to reality and face consequences of his actions. When Picard refused to let Kolb be, the scientist left the Nexus, going to Deneva moments before the planetary assault so the family could spend their last moments together. (TNG eBook: The Stuff of Dreams)