An evil mage of Vanyel's era. His name means "Darkness" in the Tayledras tongue.
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One major notable thing about Leareth is the fact that he looks like the identical but color swap to Vanyel, Pitch black hair to Vanyel's silver-white hair from using nodes in the earth, Eyes of Ebony to Van's silver, and his cloths cut similar to Herald's Whites, but all black instead. Even the heights were identical. This is quite important to note since Van stands for the Harelds and Leareth stands for the darkness that threatens them.
Vanyel's dream
Van has a dream of the battle he would have with the mage several times in the Trilogy. The first one is mentioned in Magic's Pawn and this is what caused Vanyel to panic (and slight confustion as Tylendel was dead) and try to escape from the his fate as a Herald-Mage. Years later, into the third book, Magic's Price, It's discovered after the first time sleeping with Stefan that the dream didn't fully left him, even though he destroyed the mage Krebain. This time it changed at the beginning and what only showed to be the same was "I am Leareth" from the figure looking identical to Vanyel.
Other mentions of Him
In Selenay's reign it was discovered that Leareth was another of Ma'ar's incarnations. This in turn connects him to Mornelithe Falconsbane