The Forgotten Tower is fourth quest in Act I of Diablo 2.
Rather than talking to an NPC, the Forgotten Tower is activated by reading a book known as the Moldy Tome.
Forgotten Tower Diablo 2 Quest Guide
In the tweet above, NintendoAUNZ shows a trailer for Diablo 2: Resurrected with the Forgotten Tower featured throughout the video.
To find Moldy Tome, you must look on a stone pedestal at the ruined chapel in Stony Field. Once you have acquired the book, it tells the story of a castle in the woods that was owned by an evil Countess.
According to Diablo Fandom, "The Tower is located in the Black Marsh, beyond the Dark Wood. There are five descending floors within the Tower Cellar, with the fifth floor leading to the Countess herself. Upon her defeat, a sealed chest will open, and spew out the treasure that is the reward for this quest. In addition to a substantial amount of gold, the Countess might also drop one or more runes."
You can also go back to the tower at anytime as it is available to be revisited. Despite being available for returning players, the treasure chest only opens when someone is trying to complete the original quest.