This crazy stunt comes from the same crew that pulled off the two-man Loatheb kill, and uses another pretty funny gimmick. This one uses a combination of Spellsteal on specific mobs, a talent called Incanter's Absorption, and a talent called Burning Determination.
The Bone Armor that is cast by the Death Knight Cavaliers was able to be spellstolen, and it absorbs 1,200,000 damage. You could get up to hundreds of thousands of points of spell damage, in theory. In practice it probably comes out to much less than that most of the time. Still, as you can see from the video, 100,000 spell damage was completely possible. Burning Determination was necessary because those same mobs like to spam Strangulate.