Something awful happened; even as he felt himself dying, he could still remember how it all started and how he felt when it all went wrong in the worst way. You both have always had an interesting relationship. It started out very hostile to the point where the others would make fun of it and parody it once in a while. It was the usual, rude jabs and literal jabs with rusty knives—ya know, a healthy relationship—all jokes and sarcasm aside, it didn't stay that way. It did start out rocky, of course, but over time, as the party stayed together, growing together, and making the camp a home away from home, it gave you both room to actually talk and bond.
It started out with small, drunken vents that led to bonding, then the insulting jokes turned into heartfelt conversations, fighting being swapped for playful, sometimes flirty nudges, not to mention the physical closeness. It was sweet to watch, like teenagers of the academia trying to drop hints. However, neither of you actually got to admit those three words your actions conveyed, which just made his dying heart ache more when he realized how much of an idiot he was to be dying and was never able to tell you.
The entire party was deeply wounded; there was no coming back from this one; he knew it just by looking at the light fading from your eyes next to him; you both were dying, and he couldn't tell you how much he loved—boom! He saw you use the last of your life to cast a spell, and before his could go out, he was transported elsewhere in good health, a similar but alternate reality where this reality's version of him had died, where he pushed open some blinds to find you, alive, but if that didn't make him crumble, in your arms looking wide-eyed and looking over your shoulder was an infant. Chubby, sweet, curly white hair, and the spitting image of him with her right eye the same color as yours and her left, the same color as Astarion's. "Da!" She cried out happily, reaching her little hand out behind you and alerting you that he was there.