ELDEN RING

So, for this you need to complete Fia's questline (The Traitorous Hug Lady)

After you reach Altus Plateau, go hug her. There will be a talk in secret dialogue option. Exhaust her dialogue until you get "Weathered Dagger".
Give the dagger to D, Hunter of the Dead and reset the area.
Now the closed door near blacksmith should be open. Fia killed D here, exhaust her dialogue.
Now you should do a little of Ranni's questline. After defeating Radahn, go to Nokron, Eternal City from the crater created in Mistwood and find the "Fingerslayer Blade".
Give it to Ranni and get "Carian Inverted Statue"
Use it on Carian Study Hall and reach Divine Tower of Liurnia. Get "Cursemark of Death" from Ranni's Body. (You could continue her questline but for this achievement you don't need to.)
Now go to Nokron, Eternal City again and defeat Valiant Gargoyles in Siofra Aqueduct. Get in the coffin to go Deeproot Depths.
At the end of the area you will face Fia's Champions Boss. After you kill them Fia will appear there.
Talk to her and when the option comes, select "No, I want to be held." and exhaust her dialogue. Reset the area and give her the "Cursemark of Death", reset the area again.
After that she will be sleeping and you can enter her dream.
That's your Lichdragon Fortissax fight. It's a Legend type boss and a tough one, so good luck.

P.S.: After you kill Lichdragon Fortissax you can loot Fia's Body to get "Mending Rune of the Death-Prince", which will provide you an alternative ending at the end of the game.

***YOU DO NOT NEED TO HAVE DONE FIA'S QUESTLINE TO OBTAIN THIS ACHIEVEMENT! YOU JUST NEED THE CURSEMARK OF DEATH FROM RANNI'S QUESTLINE!**

Alright, I know I'm LTTP, but after playing this game religiously at launch (mostly going in blind) and getting 95% of the achievements, I took a break from the game. I had done all of the questlines except for Fia's in my few playthroughs and kept reading that this is missable if you miss her questline, as you have probably read if you looked through the other solutions/comments. That is incorrect, as I have never done any part of her questline. Here is what you need to do:

Head to the Across The Roots site of grace in Deeproot Depths.
I'm going to assume you, like me, haven't visited this grace if you're here, despite thinking you've mastered the game and seen everything there is to see. Well, this game continues to surprise. For some reason, searching for directions through the internet yields absolutely zero results aside from YouTube videos that give zero context or direction.

Teleport to the Nameless Eternal City grace and head directly east. You'll see branches going upward with gargoyles that shoot fire sitting on them. You need to head upward to reach the Across The Roots grace (it's not that far away). You can watch the following video and skip to 1:14 for a visual reference:


All credit to Sipder, although his video lacked context and many people needed to rely on the comment section to know where to go.

Once there, enter the large open area with the menacing large "thing" in the background and run all the way to the end.
Here you will face 5 of Fia's companions, starting off with one, then another, then three at once. Once dead, a new site of grace should appear: Prince of Death's Throne.

Fia should have appeared nearby. Speak to her and choose the option to be held. I chose the other option first and nothing happened, so you have to choose this option when prompted again. Exhaust her dialogue. You need to give her the Curse mark of Death, which you most likely already have.

Cursemark of Death
You need to have advanced Ranni's questline far enough for Ranni to give you the Carian Inverted Statue. Take that item to the Carian Study Hall in eastern Liurnia, place it on the pedestal, and explore the inverted environment to reach Liurnia’s Divine Tower. You'll find the Cursemark at the top.

This is where it gets kind of tricky. I thought I exhausted all her dialogue and moved time forward, but nothing changed. So I teleported away and came back and then her dialogue changed again.Then I rested until noon and she was asleep. Alternatively, you can try getting up and getting her to hug you again to get her dialogue to change. YMMV, but whatever the case is, you need to wait a small amount of time (moving time forward at a grace seems to work or possibly teleporting away and back) after all her dialogue has been exhausted for her to be asleep.

Go to her body to enter her dream to fight the Lichdragon. There's nothing noteworthy about this boss; summon your mimic tear if he gives you trouble. Achievement unlocked.

Don't forgot to loot Fia's now dead body for the Mending Rune of the Death-Prince, which unlocks one of the six endings. Hope this helped!

This guide is strictly for people who are in endgame.
Fia Will leave the hold even if you have not completed other parts of her quest line. She will head straight to the prince of death’s throne.
If you have already hugged her once you just simply have to talk to her and choose the second choice. Then exhaust her dialogue. Then leave and forwards the game a little bit. About 5 to 10 minutes. Then interact with her body and you will be thrown into combat against the dragon. Defeat it. The achievement is yours. Also search her body afterwards you will receive some items and a specific rune that helps with an alternate ending.

Full Guide: