Playing with Astarion in your party will lead you to Cazador’s Palace in the Lower City of Baldur’s Gate 3. Here, Astarion must confront his former master by interrupting a ritual that will turn him into the strongest living vampire. Before you can even begin to explore Cazador’s Palace, though, you are stopped by the Sinister Door, which can’t be opened by traditional means in BG3.
When you approach the door, the narrator will exclaim that the door has a ring-sized hole in it and an ancient language is written all over it. To read the language, you need to pass a History and Investigation check, which will reveal that it’s a language Cazador forbid his underlings to learn.
Those two pieces of information are crucial to unlocking the Sinister Door in BG3.
Opening the Sinister Door inside Cazador’s Palace in BG3
To sum up, you will need a ring with the Szarr family crest on it to place into the Sinister Door but you also need a dictionary that lets you translate the ancient text on the door. Both of these items are found downstairs in the palace, which can be accessed by going through the red curtain to the right of the door.
Heading downstairs, you will first see a door dripping with necrotic magic. Unfortunately, you need to enter this room to find the dictionary you seek. Inside the room is a dead girl, Victoria, who is responsible for the necrotic magic. Upon entering the room, you will go into-turn based mode, which allows you to not take damage from the necrotic magic every few seconds.
Instead, just take one character and make your way to the back of the room and open up the wardrobe closest to the bed. Inside, you will find the dictionary. Pick it up, and exit the room, but don’t go back upstairs.
Keep going straight down the basement corridor until you reach a door labeled “Kennel” on the right. Open the Kennel door and a skeleton named Godey will talk to you. Astarion explains that Godey was his handler, which basically meant he tortured him. You need to kill and then loot Godey’s body to retrieve the Szarr Family Ring.
With both of those items in hand, head back upstairs and interact with the Sinister Door again. This time, you have the option to place the ring in the door and mutter an incantation from the dictionary. Doing so will make the Sinister Door swing open, allowing you to explore more of Cazador’s Palace in BG3.