On this page of the guide to Diablo 2 Resurrected you will learn how to start and finish the task The Forgotten Tower. The task begins when you discover the aforementioned location in the Black Marsh or read the book that can be found in the Stony Field. This is the Fourth of the six tasks in Act 1 Diablo 2 Resurrected. The location is easy to notice - a tall, ruined tower in the middle of meadows. Inside you will find a trapdoor that leads underground.
Head for the Black Marsh(located directly behind the Dark Wood). You can also find the Moldy Tome among the ruins of the Stony Field beforehand (it is visible on the mini-map). You will learn the background events and your journal will be updated. Finding and reading the book is not required to start or complete the quest.
The eponymous Forgotten Tower can be found (in a random location) in the Black Marsh. Inside, you will find the entrance to the underground. You have to comb through 5 levels of dungeons to reach the location's final mini-boss: the Countess.
Be careful when entering each level - you might immediately become surrounded by monsters that will block your way to the exit. It's a good idea to have strong area attacks and/or a town portal scroll at the ready.
In the dungeons themselves you will find a lot of barrels, sarcophagi and several more powerful varieties of enemies. The dungeon layout is random.
The Countess is located at the very end of the fifth and final level of the dungeon (the layout of this level is always similar). Make your way there slowly to prevent several enemies from following after you. The Countess will be accompanied by smaller helpers.
Defeating the Countess always randomly generates loot in the form of 2-3 runes. Upon restarting the game, you can return here to defeat the Countess again and acquire more runes (not necessarily in the same amount).
After defeating the Countess and collecting the loot, return to the camp. Talk to Cain to complete the quest. You won't get any significant rewards for completing this quest.