The Lord of Frenzied Flame ending in Elden Ring is known as the bad ending. The reason for this is that it shows your character taking on the mantle of becoming the Lord of Chaos, ready to set the world aflame. After this ending, you are tasked with burning the world down and everything in it to unite everyone and everything. This ending is one of the more difficult ones to get despite not requiring any questlines to be completed. You simply need to get to the right place and interact with the right set of fingers. Here is how you unlock the Lord of Frenzied Flame ending in Elden Ring.
Reach the sewers
The first step to getting this ending is to reach the sewers of Leyndell and defeat Mohg, the Omen. You can reach the sewers by going under the large dragon statue in the city and climbing down through the well. Follow the pathway below the surface and it will lead you to the sewers. After progressing through the area for a while, you will eventually come across the boss room for Mohg. You will now need to defeat Mohg before you can progress forward.
Meet with the Three Fingers
After defeating Mohg, attack the altar behind where he was standing at the start of the fight. This will cause the altar to lower into the ground and reveal a new area with a ton of dead merchants and some that have gone insane with madness. In the center of this new area, you will see a bunch of coffins sticking out of the walls of a long shaft. Drop down the shaft using the coffins as footholds to reach the bottom. When you hit the ground, remove all of your armor and walk up to the doors at the end. This will allow you to initiate the cutscene to get the Frenzied Flame from the Three Fingers.
After receiving the Frenzied Flame, you simply need to beat the game. Obtaining the flame will lock you out of every other possible ending. After defeating Radagon/Elden Beast, walk up to the remains in the center of the area and the cutscene will begin where you set the world aflame and get a pretty cool-looking fireball for a head.