In this guide, you will learn how to help Kolduun repay his debt in S.T.A.L.K.E.R. 2: Heart of Chornobyl and receive a rare weapon as a reward, which will become your first significant trophy in the game.
Upon reaching Zalesye, enter the building marked on the map and watch the cutscene. After the dialogue, approach Kolduun and accept the simple task.
Your task is to collect a debt from Grisha Valenok. Head to the blue marker pointing to the water tower and talk to the stalkers at the main entrance. You won't be able to enter, so go around the building on the right and communicate with the guy through a hole in the wall. After the conversation, head to the water tower, carefully passing through the anomalies located behind you.
Climb up, take the weapon and ammo from the stash, and kill all the hostile stalkers. Talk to Grisha, after which the quest will be considered complete.