From Edge of Darkness Wiki
War Against the Pure p. 183
(Note that killing animals upon the altar doesn’t count as “torturing prey” unless the character actively torments the animal. Killing humans, animal representatives of the shapechanger, or other shapechangers is a Harmony violation, however.)
Drawback: The altar suffers Essence bleed. After a number of days equal to the points spent in this Merit, the Essence begins bleeding off the altar, depleting it by three per day. If the Altar Stone Merit has •••, then on the fourth day the altar begins to bleed. (Spirits can claim this Essence only after it bleeds off — many therefore gather around it in the hopes of feasting.) Also, when an altar stone goes dry either from taking Essence or bleeding Essence, the altar must be “reawakened” with a Willpower point, or the altar will hold no Essence.