In Wrath of the Lich King it is item level 232, requires level 80 and is named Kel'Thuzad's Leggings. In MoP Classic it is item level 245 and requires level 80.
Read from the World of Warcraft client build files (JournalEncounterItem, JournalEncounterCreature, Achievement) and the Blizzard Game Data quest API.
How Kel'Thuzad's Leggings of Triumph Compares
Who It Suits
Cloth is the armor type Mage, Priest and Warlock wear. With Intellect, Kel'Thuzad's Leggings of Triumph suits Discipline, Holy and Shadow Priest; Arcane, Fire and Frost Mage; Affliction, Demonology and Destruction Warlock.
Transmog Sets
Kel'Thuzad's Leggings of Triumph is part of the transmog set Kel'Thuzad's Regalia (Warlock only). The other 16 pieces:
Built from the World of Warcraft client build files and recorded Auction House snapshots.
Where Kel'Thuzad's Leggings of Triumph Fits
Other Items Named Kel'Thuzad's Leggings of Triumph
1 other item in the current build is also called Kel'Thuzad's Leggings of Triumph. Same-name entries are usually a different rank, faction, quest stage or reward track of the same object.
Items Numbered Next to Kel'Thuzad's Leggings of Triumph
Blizzard numbers items as they are created, so the named items next to ID 47780 (47775 to 47785) were usually added in the same batch as Kel'Thuzad's Leggings of Triumph: the same quest line, zone, vendor or reward set.