Read from the World of Warcraft client build files.
How Diabolic Helm Compares
Who It Suits
Cloth is the armor type Mage, Priest and Warlock wear. With Intellect, Diabolic Helm suits Discipline, Holy and Shadow Priest; Arcane, Fire and Frost Mage; Affliction, Demonology and Destruction Warlock.
Transmog Sets
Diabolic Helm is part of the transmog set Diabolic Raiment (Warlock only), which the Appearances collection lists in 4 versions. The other 16 pieces:
Built from the World of Warcraft client build files and recorded Auction House snapshots.
Where Diabolic Helm Fits
Other Items Named Diabolic Helm
2 other items in the current build are also called Diabolic Helm. Same-name entries are usually a different rank, faction, quest stage or reward track of the same object.
Blizzard numbers items as they are created, so the named items next to ID 247136 (247131 to 247141) were usually added in the same batch as Diabolic Helm: the same quest line, zone, vendor or reward set.