Narbondel

The Narbondel was a huge stalagmite in the city of Menzoberranzan and served as a clock for its inhabitants.

Tomb of Menzoberra
Besides being the clock of Menzoberranzan, the stone pillar housed a much greater secret. Within the stalagmite, the ancestors built the tomb of the legendary founder of the city itself, Menzoberra the Kinless. From a top of the pillar, a secret stair lead to the hidden chamber where the body of the priestess once lay in a cocoon upon a huge spider's web spanning a tremendous abyss. It was the drow Zaknafein Do'Urden who found her there and took the Dagger of Menzoberra bringing it back to the city. After he did this the spider's web crashed and the cocoon with the corps fell into the depth.

Structure
At the end of every day, the archmage of the city, Gromph Baenre, cast fire magic upon the stone to heat it. This heated point climbed the stalagmite during the day and showed the drow the passing of time. At the end, the stone glowed red before it cooled down rapidly and the archmage renewed the magic. This darkening of the Narbondel was called Black Death of Narbondel.