If damage is prevented, do I still heal?
No. Flagellant must actually take 1 or more damage for you to heal.
If Flagellant takes 5 damage, do I heal more than 1?
No, just 1. But if Flagellant took 1 damage separately five times, e.g. Abyssal Assault or Arcane Barrage, you would heal 5.
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