The ritual palace chamber where the ancient vampire Cazador plans to sacrifice victims of the city in can only be reached when you open a locked Sinister Door. Getting to the final area of this Act 3 dungeon is extremely important for Astarion's questline. Only by collecting two other items in the palace will you be able to unlock the boss battle found deep within this dungeon.
Cazador's Palace is also known as the Szarr Palace found in the Central Wall section of the Lower City once you reach Act 3 of . This information can be given to you if you track down and confront several vampire spawn outside the Outer City. Remember to keep Astarion in your party to unlock unique dialogue options during encounters tied to Cazador that might prevent unnecessary combat.
The only way to unlock the Sinister Door is to read a magical incantation from the Szaar Signet Ring translated by the Kozakuran Dictionary item. These two items are found in other spots throughout the dungeon, which you can only enter by getting past the guards outside the Palace. Your party can attack the servants guarding the entrance or trick them through a high Deception check roll in .
Having a Bard or Warlock in your party can raise your chances of tricking Cazador's guards due to both archetypes usually having a high Charisma score, which affects different dialogue checks.
The Sinister Door is at coordinates (X: 70, Y: -55) within the dungeon at the end of the northern hallway inside the building. Whether the residents within the Palace are hostile to you or not, you have to interact with them to get the two items needed to unlock the door in. Choosing to free or kill Cazador's vampire spawn could make it easier or harder to collect each artifact you'll be hunting for.
The Szaar Signet Ring is held by a skeleton guard named Godey found somewhere within the Palace. Speak to the NPC Vilhelm in the main hall, then pass a DC 18 Persuasion, Intimidation, or Deception check to convince them to tell you where
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