This is the "bad ending" of the game. Once you make it to Altus Plateau and reach the Royal Capital of Leyndell, there is a well you can find in the city to enter the Subterranean Shunning-Grounds. The NPC red phantom that probably has shown up in your Roundtable area by now will give you a key to open the lower parts of that area. At the end of the Shunning-Grounds, you'll fight Mohg, the Omen King (which also grants you the Mohg, the Omen achievement).
After the fight, hit the wall behind the chest in the room for a secret jumping puzzle area. Upon finally finishing the puzzle, you'll reach the Cathedral of the Forsaken and see a large ominous door that you can't open, unless you remove all of your armor. This room leads to the Three Fingers, who will brand you with the Frenzied Flame. Now just beat the game and at the end you'll be able to interact with the Elden Ring and become the Lord of Frenzied Flame.
NOTE: Being branded by the Three Fingers locks you out of selecting the Elden Lord or Age of the Stars ending. To make sure you're able to get all 3 endings on one playthrough through cloud save backup, beat the final boss and do the first two endings, THEN go get branded by the Three Fingers for this ending. There is a Site of Grace in the final boss room, so you're free to come and go before actually committing to an ending.
If you have already been branded on your playthrough and want to reverse it, you can follow Millicent's questline to receive Miquella's Needle, which you can use in Dragonlord Placidusax's boss arena to reverse the Frenzied Flame. Follow this guide for Millicent's sidequest here: https://eldenring.wiki.fextralife.com/Millicent
How to get all three endings in one playthrough with the Cloud Save Ending Trick:
1. Beat final boss and activate Site of Grace in final room. Save and fully quit out of your game so that your cloud save is uploaded.
2. Load your save file and interact with the Blue Summon Sign for Ranni on the floor for Age of the Stars Ending (Look at the Age of the Stars solution for more info on this and a video).
3. As soon as the ending achievement pops, press Home-Button > scroll down to Elden Ring > press Options > Manage Game > Saved Data > click Gamertag > Delete Saved Data > Delete from Console > if it still shows the save, delete it again until it’s gone > press Home-Button and resume the game > it will now sync / download your save from cloud.
4. Load your save file and interact with the statue for Elden Lord Ending.
5. Repeat step 3 and download your save file again.
6. Head to Frenzied Flame Proscription, take off all your armor and interact with the door. The Three Fingers will grab you and brand you (Watch the HarryNinetyFour video below for more info).
7. Head to the Fractured Marika Site of Grace again
8. Interact with the statue once more for Lord of the Frenzied Flame Ending.
Video guide by HarryNinetyFour for more references: