Queer identity and queer romance, the politics of living in a paradise that doesn't want people like you, the confluence of mad science and cultic ritual, Sea Monsters, and especially as many variants of god-fucking, monster-fucking, and moster-god-fucking I can think of (well, almost as many).
EXPLICIT sexuality, mature themes.
Main ship and romantic storyline, Celebrimbor/Osse. Other important romantic relationships are (past, abusive) Celebrimbor/Sauron, Osse/Uinen, (past) Osse/Olwe. Imporant friendships are Celebrimbor&Finrod, Celebrimbor&Gil-Galad, Celebrimbor&Elrond. A rather important 'other' relationship between Celebrimbor and Maeglin/Lomion.
Absolutely left-field ships, polyamory, obsessive hyperfocus on the subtle repressions of queer identity in otherwise tolerate societies, really just hypercube-level mad science, an incidentally suspiciously good depiction of bipolar depression, and when I say monsterfucking, I mean monsterfucking.
Part Three: What's He Building in There?