- You may be looking for the Master who replaced him.
The Sentinel of Quan was the head of the Fellowship of Quan, a cult formed by the descendants of the miners who originally wrecked the ecosystem of Tuven III, and dwelled in underground tunnels awaiting the return of Quan.
The Master, who wanted to revive Quan for his own purposes, murdered the original Sentinel using his Tissue Compression Eliminator. Taking advantage of the Fellowship's constant wearing of hooded robes, and using a voice-filter device hidden on his person to sound like him, the Master impersonated and replaced the Sentinel, assuming command of the Fellowship. However, unable to decrypt a key inscription, he was ultimately forced to summon the Sixth Doctor to Tuven. The Doctor subsequently exposed him to Quan and to the Fellowship. (PROSE: The Fellowship of Quan [+]Loading...["The Fellowship of Quan (short story)"])