I remember this back in the days of priests picking up spell hit gear to MC Instructor Razuvious's adds. From browsing priest forums and posts, etc, it seems you need 38 spell hit rating to bring the base resist chance for level 72 mobs from 4% to the minimum of 1% (and I presume it does the same resist check every "do I break out of this now?" tick). On last week's Moroes kill, we had two adds shackled. The shadow priest's shackle never broke, not even momentarily. It was applied a total of 6 times over the course of the fight, and the add despawned as we killed Moroes and moved another out of the room. The holy priest's shackle broke 3 times (technically twice on its own; one time was the priest accidentally hitting Mind Blast instead of Shackle Undead), and resisted shackle right before stunning and killing him. He cast the spell 12 times over the course of the fight, and the only real difference between the two is that the shadow priest had enough spell hit, and the holy priest had none.
(on a slightly unrelated note, this week we brought
zero priests to Moroes, and one-shot him for the first time in our guild's raiding history. Hmm.)
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Two accounts, two tankadins! Raaar.
