A few cards in Scars of Mirrodin block (Septic Rats, Corrupted Resolve, Viridian Betrayers) refer to a player being poisoned. A player is considered to be poison if he or she has one or more poison counter. In a two-headed giant game an individual player is considered to be poisoned if any member of the team has a poison counter.

How each of these abilities work varies on how they are worded. Chained Throatseeker cannot attack unless the defending player is poisoned. This restriction matters when attackers are declared. In order for the Throatseeker to attack the defending player must have a poison counter before attacking creatures are declared.

Corrupted Resolve works a bit differently. It can target any spell on the stack. The check to see if the controller of the spell targeted by Corrupted Resolve is poisoned happens when Corrupted Resolve resolves. It is possible to play Corrupted Resolve and then give the controller of the targeted spell a poison counter (with something like Fallen Ferromancer) in order to have the targeted spell become countered.