Oracle Text

Living weapon (When this Equipment enters the battlefield, create a 0/0 black Germ creature token, then attach this to it.)

Equipped creature gets +X/+X, where X is the number of creature cards in all graveyards.

Equip 2

Card Rulings

6/1/2011 The value of X is calculated continuously as the number of creature cards in graveyards changes.
6/1/2011 If there are no creature cards in any graveyard when Bonehoard’s living weapon ability resolves, the Germ will be 0/0 and put into its owner’s graveyard.
6/1/2011 Although creature tokens go to the graveyard before ceasing to exist, they never count as creature cards and won’t increase the bonus granted by Bonehoard, however briefly.
6/1/2011 If lethal damage is dealt simultaneously to the equipped creature and another creature, both creatures are put into the graveyard at the same time. The equipped creature won’t get an additional bonus from the other creature.