This is a Tunnels and Trolls adventure in the Castle Calahony series.
Your group enters the circle while the last man watches the entrance and investigate the black hole. It turns out to be a pit leading down. The pit is about man-sized, and iron bars have been set to easy climbing.
As you investigate it, arrows fly down. They are hidden in the rocks, and only now you detect small holes in the walls.
Go to the opponents section and roll up 5 mountain orcs. Have each of them make a L1-SR at dexterity. They choose their victims at random from your group (take a pack of cards to find 5 victims). You may take hits on armour, but warriors may not double their armour.
You cannot attack them, and have no time to descend into the pit, so
fly back to the cleft. From there you return
to the camp.