Diablo II/Forgotten Tower

Unlike the other quests in Act I, this is triggered outside the Rogue Encampment and it's an optional quest. To trigger this quest, find the Mouldy Tome in the Stony Field and read it:

''"...And so it came to pass that the Countess, who once bathed in the rejuvenating blood of a hundred virgins, was burned alive. And her castle in which so many cruel deeds took place fell rapidly to ruin. Rising over the buried dungeons in that god-forsaken wilderness, a solitary tower, like some monument to Evil, is all that remains.

The Countess's fortune was believed to be divided among the clergy, although some say that more remains unfound, still buried alongside the rotting skulls that bear mute witness to the inhumanity of the human creature."

Black Marsh
Interesting, now head to the Black Marsh to find the Dark Tower, don't know how to get there? Well it's accessible from Dark Wood if you've done the "Search for Cain" quest. Whilst you're at the Black Marsh, it is recommend to look for the waypoint as well which would prove useful.

Hole Level 1
Optional

Hole Level 2
Optional

Forgotten Tower
Once you find the tower (not much of tower since it's collapsed) head into the cellars, your destination is the 5th floor of the cellar.

Tower Cellar Level 1
Don't worry too much about navigation since the cellars are relatively small in Normal, the only things you should be worried about here are the Champion and Unique monsters on every floor ranging from undead Ghosts to Blood Clan goatmen. As a general rule, bear left for the most direct route to the next floor.

Tower Cellar Level 5
Once you arrive on the 5th floor, explore around the area and open every chest to obtain mounds of gold, the Countess is located in the back room.

Countess
Super Unique Dark Stalker (Fire Enchanted)

Care for a bloodbath?

She's surrounded by her minions: pull them out of the room slowly and kill them before facing the Countess. The Countess shouldn't be too hard so the fight would be relatively quick. The quest ends as soon as you defeat the Countess.