This video walks you through the entire process.
This ending is highly missable and by far the most complicated one to get. You must complete a rather long quest line for it.
Most importantly: NEVER EVER CURE YOUR HOLLOWING WHILE GOING FOR THIS ENDING!!! This means don’t heal your dark sigil, don’t give a firekeeper soul to the firekeeper and don’t use purging stones. Hollowing is what makes your character look like a zombie. You are allowed to restore ember.
Do all of these steps as soon as possible. Don’t do anything unncessary in between and don’t progress the story any further than you have to or you will mess things up. If anything goes wrong you can speedrun the story within 2-3 hours in New Game+.
Step 1: Recruit Yoel on the bridge of “Foot of the High Wall” bonfire
Step 2: Talk to Yoel in Firelink shrine and let him give you a free level up
Step 3: Kill yourself 8 times in a row (jump off cliff)
Step 4: Level up 4 more times at Yoel (every 2 deaths = 1 level up)
Step 5: Now Yoel dies and is replaced by Yuria in firelink shrine. Talk to Yuria (fast travel back and forth so she spawns)
Step 6: Talk to Anri (a Knight) at the Halfway Fortress bonfire (Road of Sacrifices area)
Step 7: Kill Deacons of the Deep Boss (Cathedral of the Deep area)
Step 8: Talk to Anri in Firelink Shrine after defeating Deacons of the Deep
Step 9: Talk to Anri near Catacombs of Carthus entrance after killing Lord of Cinder: Abyss Watchers boss.
Step 10: Talk to Anri further down in the Catacombs (by the hanging bridge before Wolnir boss)
Step 11: Kill Horace down in the area where you fight Old Demon King boss (destroy hanging bridge to climb down there)
Step 12: Return to Anri and talk to her
Step 13: Talk to Anri at the Church of Yorshka Bonfire (Irithyll of the Boreal Valley area)
Step 14: Kill Pontiff Sulyvahn boss (Irithyll of the Boreal Valley area)
Step 15: Talk to Yuria in Firelink Shrine
Step 16: Marry Anri in the room below Anor Londo bonfire (Irithyll of the Boreal Valley area)
Step 17: Talk to Yuria one last time in Firelink Shrine
Step 18: Continue through the game and kill the final boss (Soul of Cinder). After the endboss interact with the bonfire in front of you to trigger the Usurpation of Fire secret ending. Congratulations you have done it!
Any help with issues with this trophy. I have completed this quest line 4 times in the last week. Each time completing the marriage ritual and never curing the hollow. Yet when I defeat the soul of cinder final boss and link the flame I do not get the ending. I have followed your guide and many others including videos yet to no avail. Please any help as to what could be the problem would be appreciated as it is the final trophy I need for platinum.