Soul taint
Soul taint is one of the first battle spells a young Umbra core can hold. It has been cast since the Age of the First Sealborn, when newcomers first learned to shape power through their cores, and it has changed little since.
Before the fighting starts, the spell falls on a single enemy company. It does not burn or cut. It numbs. Soldiers it touches can lock in place, unable to raise a weapon. Umbra cultivators explain that the spell lays on the living a little of the cold the dead feel at the threshold of the Deep, and the body stiffens as if it were already lying there.
Which company it finds is not the caster's choice. Like any magic in battle, it must pass the enemy's ward, the resistance of its affinity and the creature's own resistance in turn. Cast from its own affinity it strikes hardest; against the grain it is weaker, and a deeper realm strikes harder.