Morinund was an ordinary, quiet mage who suddenly went mad and murdered people. He was locked away in The Irish Sanctuary until Tanith Low accidentally freed him. Years later, he returned to her with a secret.
Moribund was a normal sorcerer who went mad and started to kill people. He was eventually captured but psychiatrists couldn't work out what had made him do it. After Tanith was mortally wounded by The White Cleaver, she managed to trap it behind a sealed door. Moribund, one of the prisoners in the complex for years asked her to release him in order to help her. With little choice she obeyed. However, he left her there and escaped, vowing he would repay her if he survived.
Over six years later, Tanith, now possessed by a Remnant, was searching for the God-Killer weapons. She planned to destroy them to ensure the vision of Darquesse destroying the world would come through. This would mean anything that could kill The Faceless Ones, Mevolent, Argeddion or even The Unnamed and could harm Darquesse would have to be destroyed.
At Johann Starke's party, she thought she recognised a face from her past but dismissed it as she was busy trying to acquire The Dagger.
Finally, when she was captured in The British Sanctuary, Moribund approached and freed her. He revealed that it was him that helped deal with Starke, Crab and Paulie. He also revealed that he was Remnant possessed and had learned to step away from violence and urged Tanith too as well.
His debt repaid, he left Tanith and Wilhelm Scream, even after Tanith asked why the shouldn't stick together. He simply stated that being possessed shouldn't limit them.
Later, Tanith started showing signs of compassion, such as helping Aurora Jane, although it may just have been to look good in front of Valkyrie Cain. While it would be difficult, Moribund's words may have been true.