You can find all kinds of weapons in Elden Ring, and the Godslayer’s Sigil is one of them. Trapped in Stormveil Keep, you will find this strange armament. Here’s exactly where to get it.
First you should spawn to the mercy place Liftside Chamber in Stormveil Castle. The castle is north-west of Limgrave, and to reach the Godslayer’s Seal you must defeat Margit the Cruel Omen and obtain a Stonesword Key.
Once you are at the Place of Grace, do not take the elevator up. Instead, head back outside where a sea of soldiers are waiting to defeat you right in front of you. You can smash your way behind their forces, or run as fast as you can and dodge incoming attacks. Go to the right side, near some trees, you will find a hallway that leads to a staircase that leads down. A few soldiers will likely follow you inside if you decide to run, so make sure you dispatch them before heading down the stairs.
At the bottom of the stairs is a room with about four to five rats. Make sure they don’t ally with you and head back to the narrow staircase to take them out one by one. When that’s done, you’ll see a door covered in fog. Here is the seal of the Deicide. On the left side of the nebula you will see a place where you can insert the Stonesword Key.
The fog will disappear and the Godslayer’s Seal will be in the left treasure chest. On the right you will also find a Godskin Prayerbook. When you use the Godslayer’s Sigil, its passive ability boosts your Godslayer’s incantations. You also need 27 Faith to use it properly.
https://www.gamepur.com/guides/how-to-get-the-godslayers-seal-in-elden-ring How to get the Godslayer’s Seal in Elden Ring