Blinding Glory. Any creature that can see the hydra and moves to within 20' of the hydra, starts its turn there, or becomes able to see the hydra while there, must make a Constitution save (DC 23) or be Blinded for one minute.

Hold Breath. The hydra can hold its breath for 1 hour.

Hypnotic Glory. Any creature that can see the hydra and moves to within 20' of the hydra, starts its turn there, or becomes able to see the hydra while there, must make a Wisdom save (DC 22) or be Charmed for one minute, as per Hypnotic Pattern. Any creature that resists this effect, or has it end on it, is immune for 24 hours.

Immortal. The hydra cannot be killed unless all of its heads are dead. Each head has its own way of killing it, discussed in the many heads trait below.

Legendary Resistance (3/day). If the hydra fails a save, it can choose to succeed instead.

Multiple Heads. The hydra has up to thirteen heads. While it has more than one head, the hydra is immune to being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.

Its heads are numbered, and each head has its own requirements for being killed; heads must be killed in the order given below, and a single attack can only kill one. Each dead head reduces max hp by 25 (to the threshold for that head being disabled).

1. Red; disabled if HP <= 325, dead if takes cold damage while disabled.
2. Red; disabled if HP <= 300, dead if takes cold damage while disabled.
3. Orange; disabled if HP <= 275, dead if takes lightning damage while disabled.
4. Orange; disabled if HP <= 250, dead if takes lightning damage while disabled.
5. Yellow; disabled if HP <= 225, dead if takes force damage while disabled.
6. Yellow; disabled if HP <= 200, dead if takes force damage while disabled.
7. Green; disabled if HP <= 175, dead if takes thunder damage while disabled.
8. Green; disabled if HP <= 150, dead if takes thunder old damage while disabled.
9. Blue; disabled if HP <= 125, dead if takes fire damage while disabled.
10. Blue; disabled if HP <= 100, dead if takes fire damage while disabled.
11. Indigo; disabled if HP <= 75, dead if takes radiant damage while disabled.
12. Indigo; disabled if HP <= 50, dead if takes radiant damage while disabled.
13. Ultraviolet(black); dead if takes necrotic damage while at 0 hp.

Reactive Heads. The hydra gains one reaction, only usable for opportunity attacks, for each head it has, excluding disabled heads. If using bites with multiple heads, it must use one reaction per head.

Regeneration. If the hydra has at least one disabled head at the start of its turn, it regenerates 25 hp and gains 25 temporary hit points. Temporary hit points from this trait can stack, and represent additional black (infrared) heads. The hydra is immune to effects that reduce or prevent healing, or reduce max hp, other than the effects of killing heads.

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

Multiattack. The hydra uses Prismatic Beam twice, uses Bites with all remaining active heads, and uses Constrict. It may use Constrict before or after Activate Head.

Prismatic Beam. Roll 1d12 to determine which head is used. If the indicated head is dead, it uses head 13. If the indicated head is disabled, it uses regeneration a second time. All beams are 5'x100' lines; creatures hit struck by the beam must save (DC 23, type varies) or take 10d6 damage, or half as much on a success. Effects by color

• Red: fire damage, Dexterity save.
• Orange: acid damage, Dexterity save.
• Yellow: electricity damage, Dexterity save. On a failed save, character cannot take reactions until the start of their next turn.
• Green: poison damage, Constitution save. On a failed save, character is poisoned for 1 minute, but may repeat the save at the end of every turn.
• Blue: cold damage, Constitution save. A target reduced to 0 HP by this ray is Petrified.
• Indigo: psychic damage, Will save. On a failed save, character is confused for 1 minute, but may repeat the save at the end of every turn.
• Ultraviolet: force damage, Strength save, a target that fails its save is Restrained but may spend its action to remove this condition. Until the hydra's next turn, the path of the beam is a gate to a plane of the hydra's choice; any creature that ends its turn in the path of the beam is pulled through.

Bite. Melee Weapon Attack: +15 to hit, reach 15', one target. Hit: 7 (2d6) per head used (max 3). Its ultraviolet head does 15(2d6+8).

Constrict. Melee Weapon Attack: +15 to hit, reach 15 ft, one target. Hit: 17 (2d8+8) bludgeoning damage. If the target is Huge or smaller, it is Grappled (escape DC 23). Until this grapple ends, the target is Restrained and the hydra cannot use Constrict on any other target.

Absorption: On being missed by, or saving against, damage of the type of a disabled head. Heal by the damage the attack would have done.

Refraction: On being missed by, or saving against, damage of a type corresponding to an active head. Change the target of the attack, as if the source of the damage was the hydra; if the source was an area effect, the entire area is moved.

The hydra can take 3 legendary actions, using one of the options below. It can take only one legendary action at a time and only at the end of another creature’s turn. The hydra regains spent legendary actions at the start of its turn.

Prismatic Beam (Costs 2 actions): The hydra uses Prismatic Beams with a single head.

Detect: The hydra makes a Perception roll.

Rainbow Leap: The hydra teleports up to 120' to a location it can see.