This set is sold by the following vendors:
Alliance: Arcanist Uovril at the Silver Enclave in Dalaran
Horde: Magister Arlan at Sunreaver's Sanctuary in Dalaran
Neutral: Goodman the "Closer" in Icecrown Citadel.
The legs and hands armor also drops from Koralon the Flame Watcher in the 25-man version and Toravon the Ice Watcher in the 10-man version of Vault of Archavon.
Relentless Gladiator's Regalia is the Arena Season 7 Set. It contains 5 pieces. Has set bonuses at 2 and 4 pieces.
Relentless Gladiator's Silk Amice
Relentless Gladiator's Silk Cowl
Relentless Gladiator's Silk Handguards
Relentless Gladiator's Silk Raiment
Relentless Gladiator's Silk Trousers
Wearing more pieces of this set will convey bonuses to your character:
2 pieces: Increases Intellect by 25.
4 pieces: Increases the damage done by your Arcane Blast, Fireball, Frostbolt, Scorch, Pyroblast, Arcane Missiles, and Frostfire Bolt spells by 5%.
4 pieces: Increases Intellect by 75.
Here you can buy Season 7 Mage Relentless Gladiator's Regalia for EU or US Servers.
You'll get guide for getting Season 7 Mage Relentless Gladiator's Regalia
