Overview
La Cage au Fou is a main quest in Blood and Wine. After the events of Blood Run, Geralt meets Regis in his crypt and helps him brew a potion to find Dettlaff.
Regis’ crypt
Regis lives in the crypt of the Mère-Lachaiselongue Cemetery, but he does not answer the door, so Geralt has to find another way in through a neighboring tomb full of kikimores. Burning their eggs brings out a kikimore warrior. An illusory wall that the Eye of Nehaleni reveals hides the sword Casus Foederis.
After a warm reunion, Geralt shows Regis the severed hand. Regis can brew a resonance potion to track Dettlaff, but it needs rare ingredients. His ravens bring word of a spotted wight nearby, a creature Geralt thought was extinct.
The wight
At the Trastamara Estate Ruins, spoons are scattered everywhere, barghests roam the grounds and curses are scratched into the walls. Take care: passing Doren Alma Estate on the way can trigger Extreme Cosplay. Examining enough letters, diaries and tables in the house tells Geralt how to lift the curse. A spoon key here starts Spoontaneous Profits!.
Geralt hides in a cupboard until the wight comes to its cauldron. He can attack it, or try to lift the curse. Share a meal with it and eat without spoons. Lifting the curse gives Geralt wight brew and reveals that the wight was Marlene de Trastamara. He takes her to Corvo Bianco to recover, which later leads to The Hunger Game. Killing the wight yields its saliva gland instead.
Tesham Mutna
The last ingredient requires Regis to give in to his bloodlust. At the Tesham Mutna ruins, a secret entrance leads to an ancient vampire sanctuary, and the chests along the way hold the whole Tesham Mutna armor set. You will not come back here, so loot everything. Geralt lays scavenger bait, and waves of necrophages, fleders and katakans attack. Keep Quen up, because exploding scurvers hit hard. After about ten kills, Regis’ transformation provides the last ingredient.
The vision
Back at the crypt, the potion shows a vision of Dettlaff meeting Count Louis de la Croix and the count’s death. The next lead is a bootblack near Beauclair Port, and the story continues in Where Children Toil, Toys Waste Away. The side quest Amidst the Mill’s Grist is only available until that next main quest is finished.