Updated: January 29, 2022
This item can be found in the Deathbringer Saurfang’s Cache after defeating him and the other end-wing bosses (Professor Putricide, Blood-Queen Lana’thel, Sindragosa, and The Lich King) in the 10-man Heroic or 25-man normal version of Icecrown Citadel. You can then turn it in to Rueben Lauren in Dalaran for a set of item level 277 tier 10 armor for your class.
Note that only DPS classes can get a full set of tier 10 armor this way. Tanks will want to wait for the other tier armor upgrade options.
If you have a full set of tier 10 armor, you can upgrade it to heroic using the Mark of Sanctification, which is purchased from the Rogue gear vendor in Icecrown Citadel with Emblems of Frost. It can also be bought with a Vanquisher’s Mark of Sanctification, which is obtained by defeating the end-wing bosses in the 10-man Heroic or 25-man Normal version of Icecrown Citadel. This item can be upgraded to heroic using the Conqueror’s Mark of Sanctification, or a Protector's Mark of Sanctification. Binds when picked up. Requires Warrior, Hunter, or Shaman.