Ancient Tower Study is a level within Ancient Tower found on the Greyhollow Island in Act V of Diablo III, in Adventure Mode only.
It is accessed via Ancient Tower Passageway. Once inside, an unmarked event starts: players can touch the tome of the High Cleric (getting a portion of High Cleric's Journal). This will summon the Cleric's spirit, who will thank the player for willingly sacrificing their life. Before any further notice, he will summon Fharzula.
Killing the demon will open passage to the Ancient Tower Battlements.