There’s a cap to how many monsters can be on a level (I think 50). A few Legendary Lands are at that cap from the get go.
Viewing post in Caverns of Xaskazien II comments
Yeah, figured as much. I noticed it poped up after I summoned a bunch of monsters. You should change the text displayed to better convey this, I thought I was softlocked bc I was playing a Warlock and couldn't gain faith this way. Something like 'It's too crowded in here for more monsters!' would be enough.