A guide to marriage…
Starting the Questline
You can start the quest line to begin this achievement by Talking to Ranni at the top of the tower at Ranni’s Rise. After exhausting her dialogue, head downstairs and talk to her three servants.
Afterwards, go back up the elevator and talk to Ranni again (This will remove the invisible wall so you can leave the area).
Head down to Siofra River Well Depths (Located south of the Third Church of Marika) and proceed through the area until you come across Blaidd. You will see him on the hill near the stairs.
Exhaust his dialogue and then return to the three sisters and talk to Seluvi at his tower. He will give you a letter that you have to deliver to Sellen. Sellen is located in the Waypoint Ruins Cellar.
Afterwards, return to Blaidd and he will tell you to meet him at Redmanes Castle.
After the boss fight, Head west from the Fort Haight Grace and proceed down through the crater to enter the city of Nokron.
Finger Slayer Blade
Once you arrive in Nokron, proceed through the area and locate the Finger Slayer Blade. It will be located underneath the big guy sitting on the throne.
Afterwards, return to Ranni and she will give you the Carian Inverted Statue (You can use this item at the Liurnia’s Divine Tower but it is not required for the achievement). After exhausting her dialogue. Leave the area and come back and Ranni will have disappeared and there will be a Grace to replace her.
The third tower in The Three Sisters Location will now be unblocked.
Climb to the top and take the teleporter. After you teleport, there will be a Key Item (miniature Ranni doll) right infront of where you spawned.
After picking up the item, rest at the grace and exhaust Ranni’s dialogue.
Continue through Nokstrella until you reach an elevator that leads downwards. Here proceed through the cave filled with the stupid death frogs and you will be invaded by Blaidd. Kill him and Ranni will thank you and reward you with the Discarded Palace Key.
This key will unlock the chest beside Rennala at the Grand Library of Raya Lucaria. The chest will contain the Dark Moon Ring.
Lake of Rot
From where you killed Blaidd, proceed to the building on the right and it will take you to an elevator that brings you to the Lake of Rot. Here you can either bum rush and chug jug through or you can navigate through the small islands which contain switches that raise more structures to prevent the scarlet rot.
Near the waterfall, there will be a coffin which you can interact with. It will teleport you to the next location. Here you will fight the next boss.
Make sure you have the Dark Moon Ring at this point.
After completing the boss fight, head up the lift and it will take you to the Moonlight Altar. Proceed towards the Cathedral of Manus Celes infront of you and jump down the hole and it will take you to where Ranni is.
Put the ring on her and you will receive the Darkmoon Greatsword. This is the end of her quest line.
Continue through the game and at the end, you will be able to summon her triggering the Age of the Stars ending and Achievement.