One of the new wrinkles in 5.5E are monsters that don't die when you kill them. These five monsters take more than hitting 0 ...

The Arch-Hag can also use its Reaction to cast Counterspell ... Another Celestial monster with a high CR in the 2025 Monster Manual is the Empyrean, a divine descendant of the gods.

The new Dungeons and Dragons book powers up high-level creatures as well, with streamlined attacks, revamped Legendary Actions, and terrifying bosses like the CR 21 arch-hag and the CR 22 ...