The Swanmera!

 

Swanmera
Large monstrosity, chaotic neutral

Armor Class 15 (natural armor)
Hit Points 114 (12d10 + 48)
Speed 30 ft., swim 40 ft., fly 50 ft.

Attributes 

Str 18 (+4) 

Dex 14 (+2)
Con 18 (+4)
Int 5 (−3)
Wis 12 (+1)
Cha 10 (+0)

Saving Throws Dex +5, Con +7
Skills Perception +7
Senses passive Perception 17
Languages —
Challenge 5 (1,800 XP) / Proficiency Bonus +3

Traits
Multi-Headed Awareness

The swanmera has advantage on Wisdom (Perception) checks and on initiative rolls.
It can’t be surprised unless it is incapacitated.

Aquatic Grace

The swanmera can breathe both air and water.

Reactive Heads

The swanmera has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, or knocked unconscious.

Actions
Multiattack

The swanmera makes three attacks: two with its beaks and one with its wing bash.

Beak

Melee Weapon Attack: +7 to hit, reach 5 ft., one target
Hit: 11 (2d6 + 4) piercing damage

Wing Bash

Melee Weapon Attack: +7 to hit, reach 5 ft., one target
Hit: 10 (2d4 + 4) bludgeoning damage, and the target must succeed on a DC 15 Strength saving throw or be knocked prone

HONK! (Recharge 5-6)

One of the swanmera’s heads unleashes a thunderous honk in a 30-foot cone.

Each creature in that area must make a DC 14 Wisdom saving throw:

• Failure: The creature is frightened of the swanmera frightened for 1 minute
• It can repeat the save at the end of each of its turns
• While frightened by this effect, a creature has disadvantage on attack rolls

Lightning Breath (Recharge 5–6)

The swanmera exhales lightning in a 30-foot line (5 ft. wide).

Each creature in that line must make a DC 15 Dexterity saving throw, taking:

• 31 (7d8) lightning damage on a failed save
• Half as much on a success

Notes 

• Swanmera are terrifying protectors of druid glades and sylph ponds, created by the most powerful members of the fey and sylvan courts who take three swan eggs, one from different nests of blessed swans in order to enchant and fuse the essence of life within the eggs to form the Swanmera. Its appearance is that of three swan heads joined in a large swan body, honking, hissing, and pecking at targets.
• The beak of Swanmera is hard and pointed with the striking power of a greatsword. It can split armor and counts as a magical weapon in order to strike and kill fiends. Lesser enemies are also subject to the powerful HONK that a Swanmera's three heads can release, making targets tremble in terror. The Swanmera are born with an innate breath weapon and are able to channel bolts of lighting through the center head of the trio.
• Although the Swanmera is born with an innate desire to protect the glade or spring that they were born at, the chaotic energies harnessed in their creation can at times make them a threat to their allies. It is best to guide the Swanmera to the direction of the enemy and then stay out of the Swanmera's way. Until there is an enemy present, most gladewardens advise apprentice druids or lesser fey to steer clear of a Swanmera as it patrols an area.