Elven essence is a racial stone which allows the player to play as an elf. Elves can not be monks or any hybrid classes which requires Monk (such as Gladiator.) The following monsters drop elf essence at the following rates (according to Spy skill.)
Elven Mage drops it at 0.001%
Elven Bladeweaver drops it at 0.001%
Elven Warrior drops it at 0.001%
Elven Ninja drops it at 0.001%