From all the information I've been able to find on it, they were closest to Elves in terms of resemblance, but it's all fairly general information. As far as if they're in existance anymore all the known Luethra were finished off by the Gorbesh in ancient times, as the Luethran Empire was declining while the Gorbesh one was coming up.

Jerecis Luyovathe, Fist of the Empire
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