A Waking Nightmare

Once you hit insight 80, you can speak to Bertil in Suomi (White Magic Master) to start this quest.

She gives you an Exquisite Letter. Read the letter, then talk to her again.
She got a letter from Marlin and you confirm it is you Marlin speaks of. He mentions an alchemist and begs to get your assistance. Bertil asks you to speak to Carson.
Head to 62,41 Suomi Village to speak to Carson.
He mentions dreams of a pure maiden who carries the secret to life itself.
Head back to Bertil, she says we need a powerful elixir to enter his dream. Fetch a Scorpion's Sting, Succubus' Hair (Mileth Crypt 23+), Raw Honey and an Ainmeal Ioc Deum (these can be crafted through Alchemy, or gained from donating to the Mileth altar).
Head back and she will brew a Sleeping Draught for you.
BE GROUPED - when you use the Draught it will port your group.
Defeat the Living Chaos with your group. (This fight has been updated since I did the quest video, it now summons spawns which you will need to kill before continueing to fight the Living Chaos).
Once you've defeated the Living Chaos, you will receive a Fragment of Chaos.
Speak to Bertil, she advises you to take the Fragment of Chaos to Rella in the Temple of Choosing to purify it.
She thanks you, you will receive 100000 experience, your insight 80 Nightmare armour and the legend mark 'Survived a Living Nightmare'.

NOTE - adding video of changed Boss fight, as it was AFTER I did the original video.