You can and probably should save this until right before the end of the game because it requires some of the relics you get as part of the main story anyway. Credit to the devs for pointing me in the direction of the Cuanacht hidden location and some of the tombs.

You need to go to three hidden areas, plus unlock five tombs, where you'll get a bunch of plot info mostly, but in the final hidden area you get an item that lets you choose this ending at the end game as long you get the information from clearing all the tombs as well. I believe you can do the first two hidden areas in the Horns of the South and Cuanacht in any order but the one in Forlorn Swords will be locked until you get the other two. Tombs can be done whenever you have the appropriate relics as well

After accessing Excalibur go to the Archspire and open the door there, you'll have a conversation inside.

After accessing the Shield of Palamedes go to this location, open the door, and have another conversation. You should also unlock the Bleeding Heart achievement in here.

Finally go to this location in Forlorn Swords (behind the waterfall) open the door prepared for a boss fight this time. You'll get an item from this to use in the end game conversation which unlocks the ability to choose this ending provided you get the info for clearing the tombs as well.

After accessing Excalibur you can open this tomb right near where you start the game. After you complete the quest inside and leave you'll be met by a Pict woman who will give you info as you progress and the final info needed for the ending after you clear all the tombs.

After accessing the Shield of Palamedes you can access this tomb in Cuanacht.

After accessing the Shield of Palamedes you can access this tomb in Cuanacht.

After accessing the Crown you can open this tomb in Forlorn Swords.

After accessing the Crown you can open this tomb in Forlorn Swords.