Elves are taken in by other Shadar-Kai or Drow at a young age and taken into Frostmoot for assassin training before they become a danger for the rest of society due to their violent outbursts. They are often seen in society riding wyvern during the day. They are usually seen as a bad omen. Someone paid for them to be there: Death or the taking of a young elf is about to happen. If they are seen in this form it is because you are not their target and that they have let you see them. Assassins use different means to conceal their real appearance.