This puzzle is straight from the second act of Baldur's Gate 3, and getting to it is not so simple. At least, you'll have to break through hordes of enemies to finally reach your goal and obtain the coveted Spear of the Night. We've made a small guide on how to solve the last puzzle — it's surprisingly the one that raises the most questions.
In the «Call of Shar» location, after passing the three trials of the Goddess herself, you will gain access to the Library. You'll need to clear it out in classic fashion, reach the area with a small pedestal, and, avoiding the floor trap, begin to solve the puzzle. Clicking on the pedestal itself brings up a window where something can be inserted, and the text of the puzzle is heard: What can silence the Song of the Night?.
The answer is not entirely obvious: it is hidden in one of the bookcases, which you first need to disarm, otherwise, you'll lose precious health. In such a bookcase, you need to find the book Teachings on Loss: The Night Singer and read it. From the first sentences, this book gives us the answer to the puzzle: it's Shar.
Return to the pedestal and insert the book into it. A passage will open, in which you can find not only the Spear of the Night but also the long-awaited Dark Justiciar's armor.