Thaerin Olhkhan was the last Blood Moon Elven monarch who get executed in 1683 for Necromancy (wich was banned since the year 783) and thus ending the Monarch line for ever. He came back as an Archlich in 1695 and started the Death Moon Wars. It is unknown when these wars ended, if they even ended. Snow Elves believe the Death Moon Wars have caused the Moon Elves to disappear.
Just like a regular Lich he has no more skin, muscles or hair, all that remains is his skeleton. Instead of a cape, mantle or veil, Thaerin chose to use a uncommon armor that was enchanted with magic tripling his magical power and thus overpowering the magic of the High Moon Elves causing them to lose territory quickly.