Honestly, the final stretch of 'The Last Olympian' left me a little wrecked — in the best, most invested way. If you just want the big, defining losses from the finale itself, the two central ones are Luke Castellan and Kronos. Luke makes the heartbreaking, heroic choice to reject Kronos and sacrifice himself to stop the Titan, and Kronos, as the invading force inside Luke's body and later in his assembled form, is ultimately defeated. Those two deaths are the emotional anchor of the ending: one is very personal and tragic, the other is the conclusion of the massive threat that has driven the series.
Beyond that core, the battle of Manhattan is brutal and there are a number of named and unnamed casualties. Charles Beckendorf, a son of Hephaestus who I’d always pictured with greasy hair and a sparks-in-the-eyes grin, dies during the final conflict — his loss hits the camp hard because he’s such a good, steady pal who gave everything. Silena Beauregard’s storyline is also heartbreaking: she’s revealed to have been working covertly and ends up killed during the course of events, having made a noble choice that complicates her earlier betrayal. Those names are the ones people tend to remember and mourn the most in the context of the finale.
If you widen your scope to the whole series, there are other important deaths that aren’t in the finale but still shape the narrative: Bianca di Angelo dies in 'The Titan’s Curse' and that moment reverberates through the later books, especially with Nico. There are also lots of unnamed demigods and monsters who fall — the final war isn’t clean or painless. I think part of why Riordan’s writing works here is that loss feels real without being gratuitous; friendships and sacrifices mean things afterward, and the characters have to carry those memories.
I always end up rereading the last chapters and feeling oddly uplifted and sad at the same time. If you’re re-reading and want to brace yourself, keep Kleenex nearby and maybe read the epilogue slower than you think you need to. There’s closure, but it’s honest: victories cost people something, and that cost is what makes the ending stick with you.