For this you'll need to enter the Hall of Echoes, which is actually much more complicated than just going through a door. In order to talk about this there may be some spoilers, so beware. Note everything you'll encounter is level 20+ or higher, and you can technically do these in whatever order you want.
First up is this one.
This is for getting the scroll that has the spell to reveal the door. After you get your battle tower, if you go through the door behind the throne, and then use the elevator to go down, you'll find your bedroom or whatever. On the table (should be sparkling) is a scroll, just pick it up for this.
Next up we have to meet somebody.
For this, you need to enter the primordial cave. This cave is behind the water near the knight statue (near the grand knight shrine). If you jump in the water from the shrine, turn left, and then go straight you should see the waterfall in question. Talena will say about about a powerful being if you are near the right one.
Keep taking the right path at forks until you reach three dragon statues. They'll each give you a quest to talk to some trees and get some seeds. You get these from trees in the Fjords, they are marked on the map. Each one has you do an incredibly easy quest to get their seeds. The only one to speak of is Irminsul as he gives you a riddle, of which there are three possible canidates (these riddles/answers come from the Divinity 2 wiki).
"Strange creatures, we cannot speak, though we have mouths and broad bellies. We sail on ships crowded with kin. What are we?" 4. Amphora
"It swings by his thigh, hangs below the belt, a hole in its front end, stiff- set and stout it swivels about. Levelling the head its wielder hoists its high because it is his will to fill a well-known hole he's oft filled before. What is it?" 2: A key
"It eats tales, wolfs down words. A weird diet that may seem - the songs of man, his chants of glory - and yet, after the feast, the devourer is none the wiser. What is it?" 1. A bookworm
Once you have the seeds from all three trees, go back to the statues, give them each the seeds, then go through the door they guarded and follow the path to find the Patriarch for this achievement.
Next up we need a shield because reasons.
For this you need to make it to the city of Aleroth and do some stuff. To get to Aleroth, you need to ride a zeppelin, to ride a zeppelin, you need parts. If you talk to Page, the guy in the champion harbor (this is the dock you'll see with a dragon shield east of the grand knight shrine in the Fjords, north of the red hammer shrine), he will give you this quest. You need to find 3 of each type of part for the quest, but you only need one of each to get to Aleroth. I'll refer you to this map (note this isn't mine, it belongs to the creators of the website, I take no credit) : http://guides.gamepressure.com/divinityiiegodraconis/guide.a...
the "S" on the map denote places where wyvern nests are, which is where you will find the parts. After you have one of each type (there are three types) go to the champion harbor and go up to the top floor and turn the parts into Page, then talk to Sepp nearby to go to Aleroth.
Once there, pull up the map. See that scroll? Go to it. You'll find a captain there, talk to him. He will tell you to use the shrine nearby, do so and go to the Aleroth-Ministry location. You'll find a certain person and another dude in here. You'll go around protecting the two of them while he breaks down portals. The best way to do so is a ton of points in explosive arrow, as this allows you to kill both enemies that spawn in one shot. After you've destroyed them all you'll get the shield.
Next we need the item that breaks the seal on the door so we can enter.
You'll want to go to Broken Valley. You'll need to be in dragon form, then enter. Refer to this map from the same guide as above : http://guides.gamepressure.com/divinityiiegodraconis/guide.a...
G1 is the generator you need to knock down, first of all. Roll up to it as a dragon and then morph into a human and hit the switch to turn it off. The orange "1" denotes the building we need to go to next. Enter and go to the top, you'll find a guy named Williams. Escort his posse to the ground floor to get a key to the mine. East of this building is the mine, go there and enter. Explore the mine until you find the key (you'll need to explore the southern part before going to the northeast section and finally opening the locked door close to where you enter). Inside you'll find a certain person you have to kill, then you'll have the sigil.
After all that, you can finally open the Hall of Echoes. It's near the Irminsul tree you talked to earlier (scroll on map). Go up to the door and go into your inventory and use (notes from maxos or something) and then enter for this achievement.