Frostbanes are skeletal figures case in ice, but with a flame burning within their chest. Through its icy sheath, this flame burns fiercely — usually in shades of blues, but often purple or green or even crimson.
The Frozen Damned. Frostbane are powerful undead created from corpses which have been frozen for at least a century (usually within icy glaciers, although there are tales of reaping fields upon the Plane of Ice which are maintained and harvested by ancient liches patient enough to mortal lifetimes for their schemes to unfold).
Ice & Fire. Their existence is born from paradox and their mockery of life sustained through an antithetical balance of elemental forces, for the necromancer kindles within their frozen breast a magical flame. This flame slowly consumes their dead flesh, but does not affect the ice which was their death and tomb. Thus heat and frost are maintained in an eternal, impossible tension.
The frostbane thus thirsts eternally for the warmth it is denied — drawing in the heat of life and flame around it, but instantly converting it into the deathly grip of ice.
FROSTBANE
Medium undead, neutral evil
Armor Class 17
Hit Points 98 (15d8+30)
Speed 30 ft.
STR 15 (+2), DEX 16 (+3), CON 15 (+2), INT 11 (+0), WIS 12 (+1), CHA 8 (-1)
Saving Throw Dex +6
Skills Perception +6, Survival +4
Senses passive Perception 16
Damage Immunities fire
Condition Immunities exhaustion, poisoned
Challenge 6 (2,300 XP)
Proficiency Bonus: +3
Aura of Ice. Anyone within 40 feet of the frostbane suffers 4d6 cold damage per round (DC 14 Constitution saving throw for half damage) Anyone touching the frostbane suffers an additional 2d6 cold damage (no save). Anyone hitting the frostbane with a melee weapon suffers 1d6 cold damage (no save). Every 3 points of cold damage heals 1 point of damage to the frostbane.
Contingent Regeneration. If the frostbane is within 40 feet of a living creature or source of heat, it regains 5 hit points at the start of its turn. If the frostbane starts its turn at 0 hit points doesn’t regenerate, it enters a dormant state and appears “dead.” If someone approaches within 40 feet of the dormant frostbane, however, they can attempt a DC 14 Wisdom (Perception) check to notice an unnatural chill in the air. If the dormant frostbane remains within 40 feet of a living creature or heat source for 5 minutes, it will rouse from its dormancy (and begin healing from itsaura of ice).
The frostbane only truly dies if it suffers cold damage while in its dormant state (snuffing out the flame at its heart).
Heart of Flame. Frostbanes are immune to fire damage and heal 1 point of damage for every 3 points of fire damage they otherwise would have suffered.
Frostspawn. Any humanoid slain by a frostbane rises as a zombie (MM, p. 315) in 1d4 rounds. There’s a 10% chance that such a zombie will transform into a frostbane 1d4 hours later. (There may be rites a necromance could use to increase these odds by helping to kindle its nascent flame.)
ACTIONS
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 9 (2d6+2) cold damage.
Frostbanes appear in The Mrathrach Agenda.
Ooh. I can see these guys being a late game threat in Rime if the Frostmaiden.
A nitpick, but “per round” is an unclear term gameplay-wise. Better version would be “at the start of their turn, creatures within ” or “at the end of their turn”, maybe with an addendum of “when entering the radius for the first time on their turn” to prevent some mechanical shenanigans. Also, Hear of Flames damage type is unspecified.