Hammer of Wrath is supposed to work when a mob is a 20% health or less but what i have noticed while raiding is that it usually does not become available until 19%. If it is gonna be 19% then change the spell and make it 19%. I know this only sounds like 1% but that can make a difference in pvp and pve performance...

It probably means that it'll work at 20.0% or lower. So if you see 20%, they likely have 20.01% or higher.

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Maaven
Blizzard QA