Mystic Runesaber

Sheltered by chaotic arcane energy, the Mystic Runesaber has eluded capture for millennia. Now from the shadows they emerge to hunt for new sources of magical power. Tame their wild energies to gain a loyal companion and ride into battle on wings of pure arcane power.

Source
This item is available for purchase from the shop or the In-Game Store for $25 or €25. It was released on May 13, 2015.

Mount Journal
''Infused with arcane power, the Mystic Runesaber has long been hidden by chaotic nether energies. Recently they have emerged from the shadows in search of ley lines to sate their thirst for magic.''

An arcane-infused feline saddled with magical runes that focus its energies into wings of pure arcane power.
 * In-Game Store

An Arcane Ally ''Sheltered by chaotic arcane energy, the Mystic Runesaber has eluded capture for millennia. Now from the shadows they emerge to hunt for new sources of magical power. Tame their wild energies to gain a loyal companion and ride into battle on wings of pure arcane power.''
 * Blizzard Shop