Cathros, The Bone Hydra

Multiple Heads. Cathros has five heads. While it has more than one head, Cathros has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.

Whenever Cathros takes 30 or more damage in a single turn, one of its heads dies. If all its heads die, Cathros dies.

At the end of its turn, Cathros grows two heads for each of its heads that died since its last turn, unless it has taken Acid damage since its last turn. Cathros regains 10 hit points for each head regrown this way.

Head Variation.  When Cathros grows a new head roll a d10 to determine what kind of head it grows. (1-2) Yellow, (3-4), Orange (5-6) Green, (7-8) Blue, (9-10) Purple

Reactive Heads. For each head Cathros has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Wakeful. While Cathros sleeps, at least one of its heads is awake.

Multiattack. Cathros makes as many attacks as it has heads making a bite attack or its heads unique attack.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) Piecing damage piecing

Yellow - Reckless Barrage (Recharge 2). Cathros fires a small ball of energy toward each other creature  Ranged Weapon Attacks: +5 to hit, range 200 ft., one target. Hit: 10 (1d10 + 5) Acid damage acid.

Orange - Magma Spittle (Recharge 3). Cathros spits out 3 balls of lava at up to three targets. Each of those creatures must make a DC 17 Dexterity saving throw. On a failed save, the creature takes 20 (3d10 + 5) fire and the ground becomes difficult terrain within a 10ft radius of them.

Green - Toxic Haze (Recharge 4). Cathros exhale a cloud of toxic fumes in a 60-foot cone. Each creature in that area must make a DC 17 Constitution saving throw. On a failed save, the creature takes 10 (2d10) poison Poison damage and can only take an action or bonus action on its next turn.

Blue - Ground Slam (Recharge 5). Cathros slams one of its at a point on the ground within 10 feet of it. Any creature in a 20-foot-radius, 20-foot-high cylinder centered on this point must succeed on a DC 17 Dexterity saving throw or take 15 (3d10) bludgeoning damage bludgeoning, On a successful save, the creature takes half as much damage.

Purple - Devastation Wave (Recharge 6). Cathros exhales a wave of purple energy and makes a Ranged Weapon Attacks: +9 to hit, range 60 ft., one target. Hit: 10 (1d10 + 5) Force damage force. A creature hit with this then must succeed on a DC 17 Strength saving throw or be pushed back 15ft.