I love the demon set on MM's and before the original shutdown leveled with /storm, /dark and /thermal. Dark and Thermal really are made for a demon MM with good synergy between both sets, if I remember my dark was a debuff nightmare which I solo'd a bit more as my thermal seemed more team centric with his buffs. If you want something really interesting go demon/storm as it was my funnest to play, the combo of lightning storm, hurricane and tornado with my demons hurling fire and running to claw attack was such beautiful chaos