Wake up the Keeper (4) | The Heart of the Mountain Darksiders 2 Guide

After you defeat the corrupted custodian, do not forget to collect his items lying around, and also to explore the area in search of potions and other treasures. The only thing you to left for you to do is return with Karn to the central area of the foundry. Approach the dormant Guardian and wait until the last part of the heart is installed. Watch a cutscene, after which you will be given further directions.

[TIP: as I have warned you before, you cannot fast travel now. So you will have to go in search of the Guardian on foot. I suggested that you make sure, before leaving the foundry, that you explored all of the areas that are of interest to you, so that you don't have to come back later.]