GrimInterals is probably what you’re looking for.
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That devotion buff bound to Judgment will only trigger when Judgment is used for the duration listed on the buff.
Yes.
While levelling, flat and % Retaliation damage are super important as you get very little past a certain point through your mastery buff/passive skills and devotion while a prime source has the biggest chunks come in later rather than earlier. Thus you want as much additional damage as possible, sets like Perdition or Dawnguard are great for this offering a lot during the mid-game.
As someone who has levelled a character from 1 to 100 with Retaliation, I would heavily recommend against it if you don’t have any twink sets like I mentioned above to provide boosts along the way or you really enjoy the tank playstyle and are patient. Without the boost you need, you’ll find yourself clearing crowds of enemies and Bosses slowly whilst being incredibly (perhaps even overly and unnecessarily) tanky in return.