You should watch:

Skin of Evil

This was episode number 22 of season 1.

The following characters appeared in this episode:

(voice)Voice of Armus
(voice) (uncredited) Enterprise Computer
Armus
Ben Prieto
Captain Jean-Luc Picard
Counselor Deanna Troi
Commander Wiliam T Riker
Doctor Beverly Crusher
Lieutenant Tasha Yar
Leland T
Lieutenant Geordi La Forge
Lieutenant Worf
Lt Commander Data
Medical Technician
Wesley Crusher