This ability is extremely confusing and you need to understand what it actually does in order to benefit from it.
First, let's clarify exactly what happens when you Avert Harm:
- For 6 seconds, you Stagger all incoming damage.
- Everyone who remains with 10 yards of you takes 20% less damage from all sources. (100,000 incoming damage would become 80,000 for them)
- The damage they did not take is transferred to you. (+20,000 damage to you)
- Your Stagger percentage is applied to this transferred damage. (with 50% Stagger, you'd take 10,000 immediately and add 10,000 to your Stagger dot)
The first point is extremely important. Remember that you usually
cannot Stagger non-Physical damage. While Avert Harm is active,
that rule no longer applies. This makes Avert Harm
a legitimate personal cooldown to mitigate Magic damage.
However,
you must remember the second point. If anyone gets within 10 yards of you during Avert Harm,
you will take part of their damage, and this has no target cap. This means
if you use Avert Harm while a large number of people are within range and taking significant damage, you will probably instantly die.
Despite these flaws, if you know you're isolated from others (for example, solo tanking adds, or on the far side of a boss with a huge hit box)
you can use Avert as a clutch CD, especially combined with immediately Purifying, to nullify large incoming Magic damage.