Well, the black knights have 30% phys resistance, demonologists do not. However, when you free moro dark, you get the shard of darkness (they have translated it splinter of darkness, which is kinda strange) and suddenly you get a tank with 30% phys resistance that can retaliate to each attack and gets improved crit with any hit. Top that up with target and stone skin and executioners can't match them at all.
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