Glorfindel was slain by a balrog in the first age, I believe, but is seen in LotR, saving Frodo from The Nine and taking him to Rivendell. I’m a bit confused about how he came back from the Halls of Mandos after his death, because if the Valar could resurrect Elves then how come they don’t bring back some of the greater ones, i.e Feanor or Fingolfin.