It looks like the shared damage effect originating from Tyrannical Curse's primary target is either not benefiting from the player's stat ratings or is having some other issue.
E.g.
[Combat (Self)] Your Tyrannical Curse deals 2281 (4561) Necrotic Damage to Target Dummy.
This is at lvl 80 with 66k+ Armor Pen vs. Target Dummies in the Stronghold. I have also noticed this issue when using the Daily in the field against "live" targets.
Sacrilege - Warlock
Contagion - Cleric
Testament - Wizard
Pestilence - Ranger
Dominion - Paladin
NIGHTSWATCH
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