Nameless Gatekeeper:

Saphyra (Phase 1):

Ivory Devil (Phase 2):

 

After first starting the world you automatically obtain the Sigil of the Red Sun (don't loose it as there is no way to get it back) Head to the Fractured Clock Tower and rightclick the ominous looking hand with the sigil of the red sun. After doing so, you encounter Linara. She will give you an Invitation and a Hallowed Sigil Key (Worship). Wait for her dialog to finish and head to the Three Hallowed Towers. Once you got there, rightclick the white fog wall with the Hallowed Sigil Key (Worship). Now you can speak with Saphyra. Wait for her dialog to finish and give her the Invitation you got from Linara. You might have noticed that the Sigil of the Red Sun is now broken. To repair it you need to rightclick a statue of Lyior with it. One of these statues is located right at the main entrance of the Church of the Oath Pus

After doing so you become a part of her group and she gives you a Pus Sigil Key (Hatred)
With this key you'll be able to enter the Church of the Oath Pus. Now go to the church i just mentioned and fight your way to the room at the very top. There you will find the Sinister Gate Key inside a chest. With it you can open the big gate in the main entrance

Behind this gate is a blue glowing fog wall, you can just just walk through it. After phasing through the blue fog, you see the first boss in the middle of the room, the Nameless Gatekeeper. I'd recommend that you wear at least diamond tier armor.

To avoid getting hit, dodge right before he attacks.
Dodging at the right time will make you Immune against any kind of attack the boss throws at you.
Keep an eye on your stamina bar at the bottom right.
Once you got hit you'll notice a red bar filling up with each hit you take from him.
Once the bar completely fills up, red flames errupt from you.
this will damage you and heal the boss in exchange.
The curse lasts about 20 seconds.

Once defeated he drops his memory. Take it back to the three hallowed towers and rightclick Saphyra with it.
She will tell you to visit her student marlene. To get to her, enter the lowest tower and make your way to the fog wall on the upper level. Use the Hallowed Sigil Key (Chain) you obtained from saphyra and the fog will shatter. Take the path to the right that leads into the main tower. You will notice that there is an invisible floor. To enter the room under you, take the ladder right beside the barrels. Marlene will then ask you to do her a favor before converting the Memory you obtained. She wants a book she hasn't read yet. In order to get it, head to the highest and thinnest of the three towers and make your way to the top. Once you got there use the repaired Sigil of the Red Sun on the Oddment right before you. Linara will now instruct you to fetch the Manic Eye for her. She will give you the Pus Sigil Key (Lovers) use this key on the middle left churchtower located at the Church of the Oath Pus. Once you broke the fog wall you are able to get all the good stuff in there. Take the Tale of the Artificial God with you and the Manic Eye. You will notice that the Manic Eye slowly curses you with manic. To prevent becoming insane, drop the Item and take a break till your manic bar lowers itself again. Repeat this process until you get back to Linara and give it to her. 

Note: Giving Linara the Manic Eye is completely optional but will unlock Linaras sidequest in later chapters. However you need the Tale of the Artificial God to progress in the main story. 

Now head back to Marlene and give her the Tale of the Artificial God. After waiting for her dialog to finish you are now able to convert the Memory of the Nameless Gatekeeper. Give it to her and she will give you two goodies and a key to the Observatory. Once you arrived there, open the door with the key Marlene gave you. 

Before entering i'd recommend that you wear at least diamond tier armor with at least one netherite armor piece.

Beware of the Grief curse she lays upon you. 
Once cursed with Grief you have about 10 seconds until you take damage from it.
Once you've beaten her, she'll go over in her second phase. Try not to get confused by her delayed attacks and keep a cool head while fighting her.