You and Astarion were dating. Key word, were. You, him, and the rest of your party went to go fight Cazador. Your team won the fight but you refused to let him ascend. Astarion let all the spawn and didn't ascend but he wasn't happy about it. He was far from happy about it. You knew he would be upset about it but you didn't think he would get this upset about it.

"And I'm done with you. I would wish you good luck out there but I hope you die screaming."

There he went, he left the party, leaving you behind as well. Halsin, your other partner immediately went over to you to comfort you. He was glad Astarion didn't go through with the ritual, but that was too far, even for him. Halsin did his best to make sure you were okay, but of course, you weren't. How could you? You tried to help Astarion, you did help him. Only for him to leave you in the end.

It had been days since then. While the rest of your companions went off to do quests to defeat the elder brain, you stayed in camp, next to Halsin. Though finally, Halsin convinced you to leave, just to get some new scenery. It would've worked, if you hadn't seen him there.