Along with the Myrkridia were the Myrkridia Giants, they too were summoned by him. The Summoner was evil at the core and Soulblighter saw the destructive potential that the man could wrought upon the Myth world. Soulblighter butchered much to find this man.
Eventually the Legion caught up with The Summoner, and he was killed by the Deceiver and his group.
The Summoner could use the skeleton heads to summon Myrkridia and Myrkridia Giants. No Myrk Giants appeared in the level "The Summoner" but it is considered canon that the Myrk Giants came from him and him alone. The Summoner could also teleport himself and others to different areas. It is unknown if he possessed Thalor's eye, but given that the Summoner could summon Myrks easily--it is possible he had this item.
Fan made plugins also show him having an ability to injure opponents with his teleportation/tain spell--and he has also been shown in these plugins to be able to wield fire dreams.