Stalker: Call of Pripyat → Quests → Hunt (Swamp Bloodsuckers)
Zveroboy gave me a quest to eliminate swamp bloodsuckers. If you have a shotgun, they're easy prey for a hunter. We kill three bloodsuckers and return for a reward of 3,000 rubles. On the other hand, at "max" I wasted a whole grenade launcher shot on them out of excessive caution. I should have simply upgraded my shotgun... And it's better to fight them individually, of course. They'll peck you to death.
Source: gamer.ru
See also:
Events. Blood Moon.
Fallout Tactics: Kansas City.
Chemical properties of NO2.