The Celestial Orb is currently the best holdable item in the game for +damage. It it available only to characters of good alignment (350 or higher). Celestial Spirits in the Air Citidel carry this piece of equipment.
This is what identify reveals about this item:
Object 'celestial orb' is type armor, extra flags glow hum dark invis magic anti-evil anti-neutral.
Weight is 2, value is 5555, level is 93.
Armor class is 3.
Worn:Takeable Hold
Affects hit roll by 12.
Affects mana by 100.
Affects damage roll by 12.