From MTG Salvation Wiki
Extort is a keyword introduced in Gatecrash, wherein it is the Orzhov Syndicate guild mechanic.  It allows players to pay additional or for spells they cast and create a small life-draining effect.
- You may pay a maximum of one time for each extort triggered ability. You decide whether to pay when the ability resolves.
- The amount of life you gain is based on the total amount of life lost, not necessarily the number of opponents you have. For example, if your opponent's life total can't change (perhaps because that player controls Platinum Emperion), you won't gain any life.
- The extort ability doesn't target any player.
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