The eternal, merciless, planes-spanning conflict between Hell and the Abyss.
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The war is fought for the Soul Harvest, winner take all; the right to receieve all the souls of the dead judged to have been evil in their lives. The uppermost layers of the Abyss and the Hells are constant battlegrounds, with continent-spanning armies marching across blasted terrains and fighting battles of unimaginable magnitude.
Occasional very powerful mortals journey to the lower planes in order to fight in the Blood War, for a variety of reasons, sometimes gaining great power but more often falling in the fray, to be added to the Soul Harvest they were fighting to possess.
If either side should ever win the Blood War, the consequences for the rest of the multiverse would be severe, as the power balance between the Primal forces (Good, Evil, Law and Chaos) would be irreperably damaged. No outcome of this sort appears imminent, however.