Stalker: Call of Pripyat → Quests → Variable Psi-Radiation
Once you've got decent gear, at least better than standard, the scientists at the base will offer you the chance to study something strange and anomalous, located in a distant tunnel in the northern part of the area. You'll be sent there with a group of mercenaries. The "strange and anomalous" turns out to be a useless artifact guarded by a horde of zombies.
However, at the tunnel exit, you'll encounter a controller. If you can't kill it quickly, shoot your former teammates; they'll quickly turn against you under the controller's influence. If you can kill it quickly... well, that's even better. And the scientists will reward you handsomely.
Source: gamer.ru
See also:
Eternia Crystal. Terraria
Fallout Tactics: Rifles
Obtaining Carb. Acids