Bloodthirst

Bloodthirst is a keyword ability. It appeared in the Ravnica block as the ability associated with the Gruul guild of the Guildpact set, and later also appeared in Magic 2012.

Description
Bloodthirst was created by Brian Schneider. It works by putting a number of +1/+1 counters on a permanent (so far only creatures) as it enters the battlefield, if an opponent has been dealt damage that turn.

Rulings

 * Bloodthirst checks for any kind of damage, including combat damage.
 * Life loss that isn't caused by damage (for example, the life loss caused by Blood Seeker's ability) doesn't cause a creature with bloodthirst to get counters, but damage from a source with infect (a mechanic from the Scars of Mirrodin block) does, even though it doesn't cause loss of life.
 * It doesn't matter who controlled the source of the damage dealt to your opponent. If an opponent was dealt damage by a source they controlled (such as Manabarbs, for example), creatures with bloodthirst that enter the battlefield under your control later that turn will get +1/+1 counters.
 * The creature with bloodthirst enters the battlefield with the +1/+1 counters already on it. For example, a 1/1 creature that enters the battlefield with two +1/+1 counters on it will enter as a 3/3 creature, not as a 1/1 creature that then becomes 3/3.
 * "Bloodthirst X" doesn't appear on any Magic 2012 cards.

Creature that grants Bloodthirst

 * Bloodlord of Vaasgoth (Vampire)