The War-Dead Catacombs are found in Wailing Dunes in East Caelid. The War-Dead Catacombs can be found to the far northern corner of the area, after having defeated Starscourge Radahn. From the site of grace after defeating Radahn, travel north until you reach the upper most corner of the area, and you'll find a small door along the cliffside.

Entering into the War-Dead catacombs you'll follow a staircase down to the site of grace. After touching it, head down the stars further and to the left and you'll find yourself in a large open room. Inside is a group of spectral warriors battling each other. these enemies will continuously fight and respawn a short time after death, making this location a good choice for farming runes.

Heading past the battling group you'll find the path that leads to the boss door, which is locked. to find the lever, head into the room with the battling soldiers from the site of grace, and run past them and turn right at a set of stairs next to the doorway out of the room. Follow the stairway up, turn right, and then turn left into a corridor, avoiding a shot from an enemy archer. Continue to follow this route as far as it goes.

At the end of this path there is a larger open room with a chair and some materials to gather. Just before this room, turn right and jump out of the window onto a ledge in a large room with a statue. Continue along this level until you find another window, with that first archer you saw just outside it. jump out and either kill or bypass the enemy, and the lever will be on the left.