At 20 stacks Sword of the Occult provides more attack damage than any item except Manamune and Muramana (if the wearer has 4500 mana or more).
A fully stacked Sword of the Occult provides the highest amount of raw attack damage of any item in the game, surpassing The Bloodthirster, Infinity Edge and Mercurial Scimitar by 30 attack damage. This does not mean, however, that it grants the highest potential damage, especially on carries who also rely on armor penetration, attack speed, critical chance, and lifesteal.
The stacking buff granted by Sword of the Occult when a kill or assist is scored is called "Blood Tribute."
Unique passive: Your champion gains 2 damage and +2% critical damage per stack, receiving 2 stacks for a kill or 1 stack for an assist. This effect can stack 20 times; you lose a third of your stacks if you die. At 20 stacks, your champion's movement speed is increased 15%.