Use these guidelines to create an original monster to challenge your characters in combat. You can build a monster ahead of time or on the fly during a game session.
To decide what CR will best challenge the adventurers, decide on the difficulty of the matchup and use the encounter building tool or use one of these shortcuts:
For a single monster against the entire party: CR = total character level / 3
For one monster per character: CR = average character level / 3, rounding up