The High King Maulgar 25-man raid encounter has an add named Krosh Firehand, which requires a mage to tank using Spellsteal to remove his Spell Shield, and maintaining aggro through modest DPS. The stolen spell shield lasts 30 seconds and reduces the damage of spells by 75%. Krosh is typically the third or fourth of the four adds to be killed prior to High King, so it is your job to hold his attention while the others are downed. Once DPS commences you must continue this cycle until he is killed, which is thankfully pretty quick. You can then join your fellow raiders in the casual Tank and Spank of High King.