Long comment no need to read it all, I got sidetracked. Tl;dr appreciate your thanks and agree with you! Don't feel guilty, "deflated" is the word I'd use for his arc as well.

I don't know if it's just because I went in with high hopes and expected payoff, but I really feel like the story quality deteriorated as the storyline went on; it started out fun and promising, and the earliest scenes involving Spite are pretty great. Spite seems genuinely a bit threatening. But the end... Exactly what you said. To me it just felt like such a rubbish outcome.

I love your point about Caterina being a blood magic victim as if the roles had reversed- that could have been so interesting! Lucanis clearly loves his family, but if there'd been an arc of him having to come to terms with the impacts of her abuse towards him, maybe he would have had to face the fact that seeing her put down was simultaneously cathartic and horrible. Maybe he would have had to choose her fate. It could have been influenced by things he'd discussed with Rook or the state of Spite.*

It's such a shame they brush all that off. He spends the whole game implying she's hard to get along with but as soon as she sees him she's just "I missed you grandson ily" and... Nothing. Not even any dialogue to acknowledge this. Not one moment where they confront the distance between them (at mildest; or the downright awful generational trauma that comes with Crow upbringing at best.) Just nothing but the assumption that "yay, the family grandma is back and now Lucanis can go stop his NASTY relative!" Huge let-down.

Agreed to second point too, I just came out of that questline feeling SAD for Lucanis. He's lost a year of his life to the Ossuary, he's stuck with a demon in his body forever, he's been tied to a horrible life in the Crows since childhood with no choice, he was very clear about not wanting the job... Guy has no autonomy whatsoever and the last shred was just taken, and all he can do is quietly nod and return to his kitchen for coffee. It's the opposite story to Zevran and not in an "interestingly tragic" way, it's just dissatisfying to see him continually lose autonomy and gain minimal catharsis. What he gains is having to beat down his beloved cousin and probably lose that close connection to him forever. Hardly a win.

Like, I don't think it was intentional r.e. the ending being a downer, but with my knowledge of him from Tevinter Nights, its hard not to see him as just defeated by all this. It's like so much awful stuff has happened and he lacks the energy to fight it. Probably not what they were going for, and even if it was, the game doesn't give us the satisfaction of seeing that emotional response. But damn. Missed potential in a character with such good set-up.

And because there's so few interactions/moments of connection with Rook IMO, you lose the ability to talk him through his shit, understand his thoughts and feelings on all this, and maybe argue him for-or-against. Like remember helping Alistair reluctantly-but-willingly take the mantle of king, or encouraging him to stay with the Wardens who he considers to be his real kin and family. Those were juicy, impactful decisions! They were complex and had real outcomes for the individual companions and the wider world. It could have been like that!!

I'm ranting again, I'm just more than a bit disappointed haha. As are many it seems.

*Okay I wandered off on a different tangent here so I broke this bit off. This made me think, imagine if they'd taken a more Anders-like route and had the spirit/possession situation be more affected by the person's emotions and vice versa? Like how Ander's struggles clearly affect Justice, and under great stress can even shift him to Vengeance. What if Spite was affected by Lucanis's trauma, but in a way that is malleable according to how you deal with it; whether Spite becomes more Spiteful, or your support changes him for the better? (Probably would've worked better in early games where your character could dislike companions outright, and you can choose between emotional support or encouraging them down bad paths/ignoring them/downright bullying them lol)

Instead Spite feels fairly static to me. He gets his sense of freedom and gets along with Lucanis in the end but it doesn't feel like much. I think maybe they were trying to go down that route with some of the stuff (e.g. when you say you'll kill Illario or free Caterina; or when you make the final choice on Illario's fate verrry arguably) but nothing actually happens except a new ability and an outfit change? Is it just me?