Medium to Gargantuan leviathan, whale , neutral

Armor Class: 18

Hit Points: 120

Speed: 0 ft , swim: 40 ft , can hover

STR

18 +4

DEX

12 +1

CON

16 +3

INT

8 -1

WIS

12 +1

CHA

16 +3

Saving Throws: Strength, Constitution

Skills: Echolocation. The Hafgufa can't use its blindsight while deafened.
Keen Hearing. The Hafgufa has advantage on Wisdom (Perception) checks that rely on hearing.

Damage Resistances: bludgeoning, piercing, and slashing from nonmagical attacks

Damage Immunities: Cold

Condition Immunities: Stunned, paralysed

Senses: blindsight 60ft

Languages: Telepathy

Proficiency Bonus: +2

Actions

Slam attack: the Baby Hafgufa uses the mass of it's body to slam into a creature and deals 1d6 + str and proficency (6)
Charging slam attack: with a minimum of a 15ft run up it slams into a creature and deals 2d6 + str and proficency (6)

Bonus Actions

Deafening whistle: The Baby Hafgufa can use it's bonus action to make a high pitched whistle, every creature in a 30ft cone has to succede a constitution saving throw or be deafened for one round.
Boom stun: The Baby Hafgufa can use it's bonus action to make a almost inaudible base note, every creature in a 30ft cone has to succede a constitution saving throw or be stunned for one round.
If used while in a body of water the cone for both effects becomes a 30ft sphere.