Naxxramas Instructor Razuvious
Click here to watch the Instructor Razuvious movie guides!
Razuvious has four adds at the beginning of the encounter in the 25-man encounter and they must be Mind Controlled by Priests (ideally with some Hit). Use Mind Sooth on the closest two and Mind Control them. Have tanks ready for the adds (taunts and quick aggro are good, but turn off auto attack if you're tanking these).
One thing I fail to mention in this video is why we use Understudies to tank Razuvious. Long and short of it: He hits royally hard and has an ability called Unbalancing Strike which will destroy tanks.
For tanks in particular, you'll notice the other mind controller had some serious issues which caused my own Understudy to take damage for too long. After mine dies and when Razuvious gets loose, I Taunt from a distance and move him back away from the healers and hit Shield Wall. You probably won't survive an Unbalancing Strike but you can certainly save a raid from a wipe in a bad scenario.