A named threat of Flooded Caverns, encountered rarely and never in numbers - real Dungeons & Dragons statistics, no auto-computed shortcuts.
Layered on top of the module's own combat AI, which still picks targets and positioning. The fight below moves through real hit-point-gated phases; every save DC and attack bonus computes from the stat block above at runtime.
He fights with the maul, armor, and shield he arrived in.
Both hands come free and a pair of claws take their place - a real weapon swap, not a new buff layered over the old kit.
Even the claws come off for one bare-handed nova and the burn it leaves behind.
CR 19 is set against Flooded Caverns's own real ambient ceiling, not auto-computed from level alone - a genuine Humanoid 5 / Barbarian 14 build at 19 total levels.
Hit points (270) follow this project's own average-hit-dice convention, and every save/attack bonus derives from the ability scores above at runtime - nothing here is hardcoded past the phase gates themselves.
Boss-flagged (bypasses the drop-rate roll, two loot-generation passes) and gated to a single spawn - only one Ironjaw is ever up in Flooded Caverns at a time.