I would personally buff its damage by 150-200% and redesign the spell to cause it to take 15% increased damage from the Warlock. Make it really feel like it’s causing catastrophic damage and makes you fear warlocks.
Redesign the legendary for Venthyr, Keep the 125% damage buff and make it deal 50% of it’s damage instantly.
Make it deal 200% damage if it hits 3 targets or less. That plus the potency conduit would fix all my issues with it.
I would just let the covenant die for warlocks, specially because the issues for venthyr on warlocks go beyond just Impending Catastrophe [who does deal catastrophic damage…catastrophically bad].
They pulled the ripcord, You can now transmog as a venthyr while being Nfae or Necrolord, who are much better spells and soulbinds.
so no real need to care for balancing covenants around multiple scenarios.
Applying aoe curses is actually kinda strong, but the ability needs to do way more in single target scenarios. Buff it per some of the suggestions above, or entirely redesign the venthyr legendary to focus on making IC a viable raid spell.
By trashing all covenant spells and making covenants 100% cosmetic based just like they should’ve been from the start.