Zardnaar
Legend
I thought one of the advantages of the bounded accuracy approach was that you could use (say) 12 CR 3 monsters to challenge a high level party. A band of CR 3 monsters doesn't need a world-shaking agenda (although they will presumably be doing more than stealing cows).
I've no idea if it works in practice, since I don't play 5e, but it seems to work for S'mon.
Can be brutal, hell hound and cr 3 spellcasters pack fireball and lightning bolt.