Warning: heavy spoilers ahead for the end of the game!!
*** Spoiler - click to reveal ***To get this achievement you must see one of the endings. There are two different endings, you can leave the room (Dark ending) or enter the kiln and take the throne (fire ending).
The final room is the Throne of Want, accessed through the King's Gate near the King's Gate bonfire. Before coming here, the previous boss you should have killed in the Giant Lord.
There are three potential NPC summons before the fog gate.
There are two different bosses that you must fight, and a third optional boss that gives you the new ending.
These all occur in the same location, you can either fight them all one after the other without much of a pause, or you can use homeward bone (or other things with the same effect) which will allow you to replenish your estus. Once you have defeated a boss the next will be there when you go back.
1. Throne Watcher and Throne Defender:
These two bosses come at you relentlessly. The Throne Watcher is nimble and has a longer reach than you might expect. The Throne Defender is larger and tougher, but much easier to dodge. They have separate health bars much like the Twin Dragonriders, and when they reach half health they will apply a resin to their weapon. Summoning a phantom can help shift some pressure off you.
2. Queen Nashandra:
Nashandra is a large dark skeletal figure with a huge scythe. At the start of the fight she summons a few orbs which inflict curse when nearby (similar to the curse urns), you can hit these to remove them (they die from any hit, no matter how weak). I suggest you take care of these as quickly as possible otherwise your health bar will diminish quickly. The easy way to beat her is to stand just outside of her scythe reach to provoke an attack where she fires a laser at you. She will raise her left hand and charge for a second, just roll out of the way then charge her, hit her until her attack ends, then repeat.
Note: if you have not met Aldia (Giant fire face guy) in the Undead Crypt or the Dragon Shrine because you skipped/missed the bonfires, and/or if you did not kill Vendrick before fighting Nashandra then the game will end here with the original ending cutscene (light ending).
3. Aldia, Scholar of the First Sin:
Aldia only appears as the final boss if you met him at the fourth primal bonfire (you will miss this if you do the Shrine of Winter skip), at the first bonfire as you enter the Undead Crypt, and at the bonfire in the Dragon Shrine (just after the bridge). Furthermore, you must have killed Vendrick before defeating Nashandra (see 'Vendrick' guide for assistance).
If the above conditions are met then after you defeat Nashandra, Aldia will talk for a bit then appear. He has fairly basic attacks and is quite weak. Do not attack him (mêlée) while he is surrounded by fire. He has a tree attack where tree spikes come out of the ground, easy to dodge. He also teleports, when he comes out of the teleport the flames are not around him, so hit him a couple of times, but they come back fairly quickly. If he charges a fireball above his head then you can attack him safely for a longer period of time, but be careful the fireball does immense damage (insta killed me at full health, 30 vitality), so once you have hit him a bit distance yourself and dodge the fireball, which has moderate speed. He also has some more attacks but they are rarer and of little consequence.
Once you have defeated him you will be presented with a choice, to either leave the way you came, or go towards the kiln. Make your choice and thetrophy should pop, along with credits that may be unskippable (I couldn't).