The Forgotten Tower lies in the Black Marsh, which connects to the Tower Cellar. There are 5 levels of cellars filled with ghosts and other creatures. It is the resting place of The Countess and her treasure.
Finding the Forgotten Tower and killing The Countess is is required to complete the fourth quest of Act I in Diablo II, appropriately named The Forgotten Tower.
Quotes
Character: This tower shall be cleansed of evil!
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