In various strategy guides and forums, I've read about the four chests you must not open to be able to collect Zodiac Spear in Necrohol of Nabudis. I wondered if there's a way to get this information in the game - like, for exmple, that it's mentioned in any of the bestiary entries.

Please note that I haven't finished the game yet (shame on me!), so please try not to include spoilers in your answer. Thank you!