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There's a lot to do in the final act of Baldur's Gate 3, but tucked away neatly in the upper center area of the Lower City is something Astarion could never forget – the mansion of Cazador Szarr. Cazador was Astarion's master while he was nothing more than a Vampire's thrall. Now free of the typical afflictions that plague Vampire Spawn thanks to the Tadpole, Astarion seeks revenge.

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But Cazador has greater ambitions than remaining a mere puppetmaster for the rest of his days, in fact, he seeks to Ascend by sacrificing thousands of his spawn at once. Whether players decide to simply kill him or usurp the ritual, the strategies to beat Cazador in Baldur's Gate 3 remain the same.
Keep Astarion Far Away Until The Fight Begins
One of the most useful tips for taking on Cazador, and indeed many bosses in Baldur's Gate 3, is to go in with as many party members as possible. If Astarion is in the party, he will be removed and placed in the ritual circle like the other Vampire Spawn, unable to participate in combat until freed. But, there's another way.
Separate Astarion from the rest of the part, and keep him far away from combat – like at the entrance to the sub-area at the top of the stairs – so he isn't automatically brought into the dialogue with Cazador. Once combat begins, just select Astarion and move him to the ritual area, where he will join combat without being the victim of Cazador's telekinesis. This prevents Cazador from buffing himself from the start of the ritual, and lets all four members of the party take part in the fight from the start.
It might feel like cheating, but players who struggle to defeat the Vampire Lord may find it incredibly useful to have the fourth party member there from the beginning of the encounter. Alternatively, players can make one of their other party members invisible right before the dialogue begins, then walk over to where Cazador will eventually place Astarion as part of the cutscene.
Then, when the Cazador fight begins, that character can simply free Astarion outright and bring him into combat. If Astarion isn't a part of your party, he will never show up, and all four companions should be available from the start of combat.