Priest · Shadow Magic · tier 5 · from level 30

Silence

Rank 1/1

Silences the target, preventing them from casting spells for 5 sec and interrupting their spellcasts for 3 sec.

Doesn't change the damage or healing of any Priest spell.

Averages, counting crits and misses, for a level 60 Priest with a typical weapon and no other gear or buffs, against a target your level. Worked out from the game's own data, the same way as the Foresight addon.