Mordack appears in fan fiction, including fan games.
In KQ2 (AGDI) he is said to be also the younger brother of Manannan and Hagatha. "Brother" in this case is ambiguous, as the members of the Black Cloak Society (unofficial) use the term as a title, whether or not there is blood connection. However, Manannan does call Hagatha his better "sibling", and its confirmed in KQ3R that they are in fact related through blood. Manannan's letter to Hagatha mentions that Mordack is awaiting Cassima's birth and planned to wait until the girl is of age before kidnapping her and forcing her into marriage.
Manannan visits Mordack on his island in the unofficial King's Quest III (IA) to discuss his plans for Alexander. A few months after Alexander's return home he teleports into Manannan's House to retrieve his brother. He then burns the house down.
King's Quest Companion ZZT
In King's Quest ZZT, Ravenlos is said to be his cousin. The games takes place following the defeat of Mordack.
Mordack is the brother of Manannan, and uncle of Valanice. He was brought back to life from the sea of souls.
Behind the scenes
He is widely considered the most powerful villain in the official King's Quest series.
In TSL it states that Mordack was in the sea of souls. However in the Guidebook its clear that the sea of souls is only for those who are good. Those who are evil end up walking the surface of the Realm of the Dead until they become zombies. Never do they have the chance to come before Samhain.
It seems that although the Lord of the Dead judges the souls of individuals, that those that fell under the influence of sins such as greed, infighting, and anxiety end up as ghouls on the surface with no chance of redemption, barred from the Underworld, to walk the surface eternally. Thus it is in the best interests of the Green Islanders to live peaceful lives.