Catacombs - Sven's Hammer (Heritage) | Side quests Lords of the Fallen Guide

You can find the hammer in one of the cells, in a secret chamber.

At the Catacombs, you can find a very interesting weapon. To find it, go to the upper part of the location [M4.15] (the location, where there are many cells and a lever for the gate to the boss). In one of the cells, there is a wall that can be smashed (on the screenshot; you simply need to run towards it (sprint) with your shield raised). Inside, there is the hammer (you can upgrade it).

The grave with a mysterious symbol.

To do that, go to the other end of the Catacombs, near the entrance to the Burnt watchtower. Locate the Mines - in the second room on the left (as you walk from the direction of the Catacombs) you will find a chest, and a grave at the wall on the left [M4.9] (there is a mysterious symbol above it). Interact and the grave will crack to reveal a better version of the weapon.