Parthar was a gnome member of the N'Vaelahr, the shadow army of Myth Drannor. At the start of the Weeping War, he accepted the proposal of the high mage Ualair to be transformed into a mezzoloth and infiltrate the Army of Darkness as a spy. The Elven High Magic spell went well and, after the Scourging of Jhyrennstar, Rhymallos was leading a regiment under orders of Aulmpiter but secretly sending precious informations to Ualair.
Then tragedy hit when, thanks to Rhymallos' information, Malimshaer discovered the Vaults of Uvaeren, a deposit of important elven artifacts. Aulmpiter personally directed Rhymallos's regiment to go to Uvaeren to take the artifacts away. Immediately, Rhymallos informed Ualair but Ualair couldn't gather reinforcements for fear of blowing Rhymallos' cover. So Rhymallos and Ualair fought alone against the Army of Darkness's troops at Uvaeren until they destroyed the vault, closing access to the artifacts forever but dying in the process.
In truth, Ualair survived but, deeply injured, survived in stasis inside a tomb in Impiltur, where he also took the comatose Rhymallos. There, in 1374 DR, Khelben Arunsun finally awoke them in order to gain their help in the raising of Rhymanthiin. At last, Khelben healed Rhymallos and finally dispelled his yugoloth body. So Parthar regained his original body.