I have a Sanctum Seeker in play and attack with four other 1/1 vampires (opponent takes the damage). Does my opponent take 4 from the vampire's combat damage and then lose an additional 4 life as I gain 4. Or does my opponent only lose 1 life and I gain 1. The card says: Whenever "a" vampire attacks, each opponent loses 1 life and you gain 1 life. Not: Whenever "each" vampire attacks...
I have a Sanctum Seeker in play and attack with four other 1/1 vampires (opponent takes the damage). Does my opponent take 4 from the vampire's combat damage and then lose an additional 4 life as I gain 4. Or does my opponent only lose 1 life and I gain 1. The card says: Whenever "a" vampire attacks, each opponent loses 1 life and you gain 1 life. Not: Whenever "each" vampire attacks...
Thanks ahead of time!
The ability triggers for each vampire that you attack with. A triggered ability can trigger multiple times simultaneously if its trigger condition as defined happens multiple times simultaneously. If it was meant to trigger just once, it would be worded "whenever one or more vampires you control attack", like Mavren Fein, Dusk Apostle (It's not often that two cards from the same tribe and the same set can be shown side by side to so cleanly illustrate a point!)
As an aside, note that the ability happens before combat damage, since it triggers and resolves in the declare attackers step, even before your opponent declares blockers.
I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Thanks ahead of time!
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As an aside, note that the ability happens before combat damage, since it triggers and resolves in the declare attackers step, even before your opponent declares blockers.
Currently Playing:
Burning Reanimator
Eldrazi and Taxes
Bug Threshold
Enchantress
Dead Guy Ale
Combo Zombies
Pyromancer Pox