You can find the box that begins this side mission by killing Stalkers after finishing Stalker of Stalkers anywhere in the game, as it will randomly drop from one of them. Unfortunately, the box is closed and can only be opened by using Taggart’s fist, which seems to have been devoured by a stalker named Henry.

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The box drops randomly from any Stalker (left). Henry is nothing but a big, Badass Stalker (right).

You will find Henry’s lair in the Hunter’s Bane section of The Highlands - Overlook area. When you first get there, some lesser stalkers will spawn and once you’ve killed them all, Henry will decide to show its ugly face. Henry is nothing more than the kinds of stalkers you have faced thus far in the game, so there is nothing to worry about in terms of new abilities. Loot Taggart’s fist that drops, then use it to open the box at the waypoint to finish.