Converted to 5e for my own reference to play The Gardens of Ynn (and/or The Land Beyond the Magic Mirror). Since I ended up geasing my party to fight the Jabberwocky I felt throwing a CR 15 version at a party of level 3 characters was over the top. This is a reduced threat version.
The Jabberwock is a strange dragon-like creature, with a long neck, bulging eyes, rodent-like incisors, and wide, grasping humanlike hands. The Jabberwock is a herald of intoxication. Where it passes, things behave drunkenly. Those who meet it are permanently altered. Perhaps there is some link to the Idea Of Thorns, but the Jabberwock is not infected.
Amphibious. The Jabberwocky can breathe air and water.
False Appearance. While the Jabberwocky remains motionless, it is indistinguishable from a large hill.
Intoxicating Aura. Observing the Jabberwocky deals d2 damage to Intelligence each round. When a 4 is rolled for intelligence damage, roll for Ynnian Alterations (no save).
Immutable Form. The Jabberwocky is immune to any spell or effect that would alter its form.
Vorpal Vulnerability. If the Jabberwocky is struck by a vorpal weapon or any weapon with the dragon slayer trait its damage immunity is suppressed for this encounter and instead gains the trait:
- Damage Resistance All damage except vorpal and dragonslayer weapons.
Multiattack. The Jabberwocky makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 1d6 + 4 piercing damage plus 1d6 poison damage.
Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 1d6 + 4 slashing damage.
Tail. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 1d6 + 6 bludgeoning damage.
Purple Haze (Recharge 5–6). The dragon shoots purple lightning from its eyes in a 60-foot cone. Each creature in that area must make a Dexterity saving throw, taking 10d8 fire/lightning damage on a failed save, or half as much damage on a successful one. A creature that takes damage from this attack is exposed to Ynnian Alterations (with a Wisdom saving throw allowed).
- Detect. The dragon makes a Wisdom (Perception) check.
- Tail Attack. The dragon makes a tail attack.
- Wing Attack. The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
If the purple blood of the jabberwocky is drunk, the creature is automatically returned to their home plane at the point they first left.