"The early Celts savored the dark side of life. They embraced war like a lover, plunging into battle naked, singing gloriously boatsful songs. They were fearless in the face of death, which their belief in reincarnation taught them was "... but the center of a long life." It was not uncommon for a man to lend money and agree on repayment in a future lifetime. Their day began at dusk; the new year at Samhain, the festival we know as Halloween. Darkness was associated with new beginnings, the potential of the seed below the ground.
In Celtic mythology and folk-lore, the wisdom of darkness is often expressed by powerful goddess figures.... Their role is to catalyze change through the transformative power of darkness, to lead through death into new life."
-Mara Freeman from the "Queen of Air and Darkness"
http://www.chalicecentre.net
I think she has an interesting take on celtic myth. The game itself uses leystones and that allows heroes to "reincarnate." This magic is something interesting i think mages and druids could both exploit. Where druids use nature magic (light), they would also have a set of skills for the dark. Mages use fire and ice, but could tap into the mysterious void. Currently, the forces are just benign but the magic casters can tap into these forces.
Mages get a netherworld mist that works like a trap on the ground beause the monster takes damage while its in the cloud. Another spell can beckon the target to the otherworld by a sort of black dot exploding around the target then imploding.
Druid: They can tap into the restorative forces of vitality and debuff health, and also debuff armor value. Also a spell that stops regenation. Maybe a set of lures for physical damage types.