But I never caught on the tragic love story of Halsin. Is that in a book lying around in the grove? I thought, I've read them all. I always thought, he and Kagha might have history. When she dies, he seems to take her death very personal. On the other hand, he never even loses a word about Nettie, his apprentice, being dead, if you kill her.
Now that I've gone back to read all the canon hints, I think that you are right. It's not explicit... however...
A video
What happened at Moonrise Towers? Halsin killed a Harper named Isobel at Moonrise Towers using the Glaive under the Grove; he thinks that the death was accidental, born from the heat of a deal gone wrong, and that she never actually posed a menace. Isobel was the daughter of Ketheric, who was the Shar Justicar who turned the Nightsong into a weapon of Shar. Halsin believes that him killing Isobel pushed Ketheric to the dark side of the Force and to unleash the Shadowcurse.
Now, this is where things get interesting... the in-game fanfic has many of the same flaws as real life ones: self-insert Gary Stu protagonist who sacrifices himself to save the day, and exaggerated versions of real characters who live in the end. In the fanfic, Balsin = Halsin, so Isobel must be Selene, the blue-eyed brunette Harper who goes to Moonhaven /Moonrise Towers and, in the fanfic's happy ending, is standing next to Balsin. I think that there's a nugget of truth in that story: Isobel was probably a Harper who tried to broker a deal between her father and the Druids.
When Isobel's parentage was discovered, the Druids might have panicked, thinking that she led them into an ambush, which is when Halsin accidentally killed her. Ketheric, in despair, might have tried to keep his daughter alive by any means necessary, even if that meant transforming her into the Nightsong, effectively splitting her from the Moon-blessed bards. This would also be an iteration of the myth of Shar and Selune, the division of one Moon Goddess into two separate entities. After all, Selene is the Greek name of the Moon.
From a romantic fanficcy perspective, Halsin might be on a quest to make Isobel whole again. Her spirit is still haunting Moonrise Towers, and the Nightsong will ask the party to release her from her curse. This mirrors the way in which the PC has to come to trust and love all the aspects of their companions, the dark and the light, without "accidentally" killing them in the process, no matter who their daddy or mommy is.
But also, if you squint, "Halsin" sounds like "Helsing". I will bet 1000 gold and two snacks for Gale that he will be part of the Cazador Heist.
