He's one of Strelok's men. They say he can sneak in anywhere without being noticed. I heard that he took out the leader of some crazy group... what was the name? Ah, 'Final Day', that's the one. They were pretty wild those boys... Oh, by the way, I heard that Ghost is doing some job for the scientists, so if you're looking for him try by Lake Yantar at the mobile lab.
Final Day is a mentioned-only faction in S.T.A.L.K.E.R.: Shadow of Chernobyl.
They only existed sometime before the events of Shadow of Chernobyl. According to Barkeep they were a group of wild stalkers, although he did not mention their armed actions.
Strelok's friend, Ghost, assassinated the unnamed leader of the group. It is assumed that the faction eventually collapsed due to lack of leadership.
This faction had a seemingly larger role in Shadow of Chernobyl prior to its release. They were referred to as Last Day. They predicted doomsday was near, and that the Zone would eventually increase its territory. They hunted zombies, studied psy-fields, and developed theories about psychotropic weaponry and their uses.