Astarion can be found standing on a road west of the crashed Nautiloid in the Wilderness. When found, a scene will play where he is looking for help in defeating an Intellect Devourer. Afterwards, he can be asked to join the party.
Astarion prowled the night as a vampire spawn for centuries, serving a sadistic master until he was snatched away. Now he can walk in the light and has the chance at a new life, but how long can he keep his past buried?
Astarion has a sadistic personality and often approves of evil, cruel or egotistic actions, though he seems to accept certain pragmatic ones. He values hedonism. He is disdainful, but not hostile, towards those he considers weak and unworthy of his attention.
Astarion likes displays of power and cruelty, and appreciates snarkiness and mocking. He is inspired when the player gains access to areas/information by deceiving others. The player does not have to be "evil" to gain his approval, however — he approves when the player makes choices that support independence and autonomy, and when the player helps certain characters in need.
Astarion dislikes displays of weakness and does not like acts of kindness or charity that offer no reward. He disapproves when the player tries to reign him in or restrict him in some way.
Players should be aware that Astarion's romance and conversations in general hold mentions of and allusions to various types of abuse, manipulation, and sexual assault.
The following section explains Astarion's romance.
Romance Spoilers This section reveals details about romance and may contain mature themes.
After the cutscene of Astarion sneaking away at night triggers, the player can find an exsanguinated boar at the bridge before the Blighted Village, to the far right path of the signpost saying 'Moonhaven' (north of the village), or just after the dialogue with Auntie Ethel at the entrance of Sunlit Wetlands (south of the village). The following long rest, the player will wake up to Astarion trying to drink their blood. He will approve of the player saying they trust him, and further approve of them letting him drink their blood. Note that if the player doesn't successfully stop him the second time the choice is presented, the player will die. They can simply be revived using a revivify scroll later. Note that players do not need to be aware Astarion is a vampire to sleep with him later on; not triggering this scene will not affect the romance.
Getting a substantial lead, either the location of the Githyanki Crèche from Zorru, Priestess Gut from Sazza, or learning about Halsin's possible whereabouts from Nettie, will trigger a scene with him later in camp when you talk to him. He remarks how he likes the player, and the player can respond in a flirtatious manner. No approval changes from any options. Note that you may want to wait to trigger this scene till later since the dialogue makes it sound as if he and the player have been travelling together for a while, but that's preference
A scene can be triggered at camp where Astarion says he has fed on a bear, the following conversation nets no approval changes but he makes a few playful quips to the player and they can do so in turn. The parameters for trigger this is unclear at the moment (possibly average-good relationship), but you need to know he is a vampire.
When you come across Gandrel, the Gur monster hunter found to the left of the Riverside Teahouse, you can progress your relationship with Astarion by allowing him to kill the Gur. In order to trigger this dialogue option, you need to have found out that Astarion is a vampire beforehand. In conversation with Gandrel, ask him what he is hunting, then choose "Only a spawn? Pity. Not like he's a real vampire." The first and second subsequent choices will trigger Astarion to reveal himself to the Gur and a fight will ensue.) After this scene plays out, you can speak to Astarion, who will then invite the player to have sex with him.
A similar scene with the same outcomes should trigger when the player reaches Good approval with Astarion, even without dealing with the Gur[Needs Verification]
Once you have had the conversation with Astarion where he invites the player to spend the night with him, the scene itself will trigger on the next long rest. If you choose dialogue options that indicate distrust, he will become upset and leave[Needs Verification]. You can gain approval with him by "playfully offering your neck." The player will wake the next morning to Astarion standing shirtless with his back turned to them; scars visible. The player can make a playful quip before asking him about the scars on his back. You can pass an Arcana check to determine the language the scars are written in (Infernal), but there are no approval changes.
As early as Chapter One, a scene can trigger where Astarion is looking into a mirror. The player may compliment him on anything to effectively “flirt” with him, aside from his moles and smile lines (this will lead to him being upset and leaving). This does not affect approval.
At either the goblin or the Tiefling party, the player can agree to sleep with Astarion afterwards. If this is the first time the player sleeps with him, it will trigger the cutscene mentioned above.
If the player has already slept with Astarion, then talking with Astarion at the party will result in a much more familiar conversation, where he can even tell the player he loves them, though he is just using shallow charm (no approval changes). Agreeing to sleep with him again will result in the dialogue choice "Astarion" becoming available once you head to bed, and the player then seeks him out. There is no additional sex scene, it's simply implied.
Heavy Chapter 2 and 3 Spoilers Ahead
Spoiler warning! This section reveals details about the story of Baldur's Gate 3.
Heading into Chapter 2 and 3, it is important to note that some choices you make, Astarion will disapprove of, but they may be for the 'better'. Disapproval of an action will not lock you out of romance scenes, and he may even change his outlook on your actions later on.
The next substantial scene with Astarion happens after the player goes to bed after talking to Araj Oblodra with Astarion in the party. She is a vendor located on the main floor of Moonrise Towers. Players can pick whichever option they feel suits how they want their relationship with Astarion to be. Picking options that manipulate Astarion into having sex with you simply for pleasure will be followed by a cutscene where Astarion breaks up with the player.
A similar scene with the same outcomes will trigger if your approval with Astarion is maxed in Chapter 2, even if you don't talk to Araj
Once the party successfully completes Kill Raphael's Old Enemy, Raphael will inform Astarion about the nature of his scars and the player can then talk with Astarion about the information revealed. It is important to ponder how you want to proceed with this, as it will change Astarion's fate and the dynamic of his relationship with the player.
At Circus of the Last Days there is a dryad, Zethino, who will conduct a love test for the player for a fee. You can ask Astarion to take part in it and you will be transported to a woodsy area and asked questions about him. You may choose to respond however you wish, but choosing two or more from 'I don't think he's ever been happy.', 'He's happy right now, look at that smile!', 'Me', 'A nice, tasty rat.', and 'Sharp, wooden sticks.' will result in Astarion being upset. Note that picking more 'genuine' options 'Freedom', and 'Forever feeling like a slave to someone', will result in an 'Astarion Disapproves' message popping up, but are still considered correct answers.
Once you have made it out of the Shadow-Cursed Lands, the player can find Astarion's fellow spawn in Fraygo's Flophouse near the Southspan of Wyrm's Crossing waypoint. If the player did not interact with them, then long resting a couple times in the area should trigger a scene where they come talk to you at camp. The player is free to choose any responses they want, but may see disapproval pop up at certain choices. His siblings will attack the party, and afterwards he will talk to the player. Again, you may choose to respond however you feel. If you tell his siblings 'You can take him' when they come to camp at night, and then choose 'I wish they had', 'I panicked in the moment!', or fail the check for 'I was only teasing you.', Astarion will break up with you.
Once the player confronts Cazador, they may choose what outcome they want. They can convince Astarion to simply kill Cazador instead of performing the rite himself, or he can perform the rite and become a Vampire Ascendant. Note that if he goes through with the rite, everyone in the cells will die. An additional note: Astarion becoming the Vampire Ascendant will completely change how he acts towards everything, including the player. He will be more controlling and forceful, more stereotypical 'evil tyrant'. Some players may be uncomfortable with such a dynamic, so be sure to pick carefully.
After the conclusion of the rite, a scene with Astarion will be triggered depending on the outcome. If he went through with the rite, the player can tell him they would like to be a vampire like him, and he can and will turn the player into his spawn. The player is, as always, free to choose whatever options they want following, but becoming Astarion's spawn means you can no longer break up with him. The option is there to try, but he will not let you go. If you choose to spend one last night alive with him (become his vampire spawn and sleep with him), you will get the option to kick him in the balls when he tells you to get on your knees. Doing so will result in him permanently leaving the party.
If he did not go through with the rite, he will tell the player he would like to show them something outside the city, and take them to see his grave. You're free to choose any options you want, but note that accepting to sleep with him will result in an extra bit of dialogue where he tells the player he loves them.