The Frozen Necromancer

From the introduction: Four thousand years ago the Duladarin star elves made the Luna Valley their home after demons ravaged their home plane. They venerated their ancestors in barrows and led a peaceful existence. Then Xancrown, a mighty plague demon who hated the Duladarin for escaping destruction, arrived and spread an otherworldly disaster – the demonplague. Elves began dying from the magical disease, first transforming into insane half-demon, half-undead horrors called valok, then slaying each other from the madness. In desperation, star elf druids crafted a powerful imprisonment ritual. On the winter solstice, doomed elven warriors lured Xancrown and his demons into caves deep beneath the heart of Luna Valley. Thirteen druids performed the sacred rites and imprisoned the plague demon in the valley’s bowels. Their efforts were too late. The Duladarin perished and Xancrown slumbered beneath their corpses.