Thaxalia

Thaxalia was a Beholder in Doomvault dungeon in 1486 DR.

Personality
Thaxalia was very furious with the Red Wizard for imprison and maim itself. If someone demonstrate to want harass the red wizard, Thaxalia agreed to cooperate hoping for revenge

History
Thaxalia was a beholder that the Red Wizards of Thay invoked and bounded to the secret complex of Doomvault under Thaymount. It was maimed of it central eye and death ray and Disintegrate eyestalks in order to not attack it red wizards' masters. In 1486 when the heroes of Daggerford arrived to it, Thaxalia possibly suggested an alliance against red wizards.

Behind the Scenes
Also Thaxalia can be convinced to help the adventurers but broke it bound curse was not easy. Also remember that it was an Aberration and evil monster, there are few possibility of a true alliance with it, more reciprocate exploitation.