In Wrath of the Lich King it is item level 245, requires level 80 and is named Siegebreaker Helmet. In MoP Classic it is item level 232 and requires level 80.
Read from the World of Warcraft client build files.
How Conqueror's Siegebreaker Helmet Compares
Who It Suits
Plate is the armor type Death Knight, Paladin and Warrior wear. With Strength or Intellect, Conqueror's Siegebreaker Helmet suits Arms, Fury and Protection Warrior; Holy, Protection and Retribution Paladin; Blood, Frost and Unholy Death Knight.
Items Numbered Next to Conqueror's Siegebreaker Helmet
Blizzard numbers items as they are created, so the named items next to ID 46151 (46146 to 46156) were usually added in the same batch as Conqueror's Siegebreaker Helmet: the same quest line, zone, vendor or reward set.