I’ve ran into this a few times, and I’m not sure how to get in besides getting lucky with a scroll of teleportation. Maybe Etheral Chains, but there’s nothing for them to hook onto.
I’ve checked the walls everywhere in the vicinity, and I can’t find a hidden door.
Drop from an empty well room on the floor above
You can also make a feather fall potion to drop down with a potion of levitation and an arcane catalyst but it isn’t always worth it.
Feather Fall is technically a spell, not a potion, and you get two uses out of it.
You can also just take the fall damage and use a healing potion to stop the bleeding and heal back, as long as you have at least half your HP or so to survive the initial fall damage.
Potion of Cleansing also stops the bleeding and eliminates all hunger at the same time. Good if you are playing the healing potion challenge, but feel like jumping into a pit.