Setaal Darkmender at Death's Rise has given you the Darkmender's Tincture. Apply it to the corpses of 10 of any of the Scarlet Onslaught units found there.
10x Scarlet Onslaught corpse transformed
How kind... how generous our lord-commander is. Where he is flowers and sunshine, I am filled only with hatred.
It is not enough to end their miserable lives... I want them humiliated... debased!
What could be more unspeakable than to be turned into that which you hate? To dedicate your existence to eradicating a thing, and then in the end to become it?
Take this tincture northwest to Onslaught Harbor. Apply it to their still corpses and watch what happens, but avoid the cathedral.