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.
Every enemy present is marked at once, each with its own five-round countdown ticking independently.
A mark hitting zero doesn't end anything - it simply resets that one target's count and keeps going.
The weakest soul in the room is dragged under and finished.
CR 20 is set against Flooded Caverns's own real ambient ceiling, not auto-computed from level alone - a genuine Undead 4 / Fighter 8 / Hexblade 8 build at 20 total levels.
Hit points (204) 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 Mordraeth Kell is ever up in Flooded Caverns at a time.