The Corrupted Priest is a side quest in Egypt (Act II). It is given by Amen-nahte, a young priest found in the Temple of Seti. He will tell you that when the Telkine came through, the temple's head priest Neb-Kemi stood against it, but the Telkine turned his spells against him and Neb-Kemi became 'transformed'. You are to search for the priest in the deserts.
You will find Neb-Kemi in a tomb on Meretseger Ridge, which can be accessed going East from Thebes and from the southern Highland Pass. Neb-Kemi is an undead sorcerer with multi-shot ranged attacks and a Deathchill Aura active, which slows down melee attackers and continuously decreases their health. He is also protected by several Sandstone Guardian Statues, which are somewhat damage resistant, but slow and will do only moderate damage.
After you have killed Neb-Kemi, return to Amen-nahte in the Temple of Seti to complete the quest. You will get experience and a random magical weapon, usually of rare (green) or epic (blue) quality.
Reward: 15 000 EXP and a Magical Weapon