Abaddon Killer

Statistics
Damage: 99 Delay: 504 Weapon Class: Great Axe Slot: Weapon Jobs: WAR Races: ALL Level to Use: 75 Rare / Exclusive  Upgraded to: Bravura 
 * In Dynamis: Metatron Torment

Relic Weapon
This is the fourth stage of the Relic Bhuj upgrades.

(This information may not be entirely accurate as there are few who have done Relic upgrades and talk about it.)

To get this horn you must have these items:
 * Dynamis Bhuj (third Relic stage)
 * Attestation of Force
 * M. Silverpiece x40

It seems as though you turn these in to a goblin named Switchstix (Castle Zvahl Baileys I-8 on first map). Unsure how long you need to wait to get the Millenium Horn back.