The Heart of restrained Thunder -  as it is called by legend - contains the Demisolar of Kord - Zephyr, the twinned soul. And although it is rumored to be an item of legend it is merely a containment ward for said being. Zephyr itself is represented by this item and the corresponding "monster" Zephyr.

Whoever calls this hammer his own and proves himself worthy of it will make Zephyr his soulmate. Kord's child will change Zephyr's life and his/her own in four trials:

The calling
If the request for companionship succeeds, Zephyr is henceforth part of the life of Kord's child. Neither will Zephyr show it's true form nor will it communicate with it's bearer.

The Care
Care will seal the bond. With the gift of life, Zephyr is an independent being and able to communicate with Kord's child. Zephyr comes into being as a small gryphhound and with merely more intelligence than an average dog. Nonetheless she already understands the words of the hammer's bearer. Zephyr is playful, mischievous, hungry and very curious. For all intents and purposes the "Lion" monster statistics are to be used.

The Rage
In times of great rage Zephyr will awaken and reveal her true power. The gryphhound Zephyr will grow in spirit, strength and size. She now can be ridden like a normal steed.  Her height at withers is around 5 ft 4 inches. Moreover she is able to telepathically communicate with the bearer. She speaks in short sentences and her nature is mostly joyful, protective and straightforward. For all intents and purposes the "Zephyr" monster statistics are to be used.

Moreover, Zephyr - the twinned soul - is now able to show her true self. As long as Zephyr - the gryphhound companion - is asleep she may choose to appear in the form of "Zephyr - Demisolar of Kord". Her appearances are usally brief, cryptic and private in nature. Only in the direst of circumstances will Zephyr appear for visitors to see and take action on the realms of mortals. Use at the DM's discretion.  

The Loyalty
For this Kord's child must perform a true deed of Kord and overcome him/herself. Kord demands strength: strength against enemy and friend, against hate and love. Kord's child faces even his/her greatest fear and conquers it.
If this succeeds, Zephyr must leave the plane of transcendence and walk beside Kord's child as a living being. The - now gryphon - Zephyr becomes mortal, gains the ability to fly, and gives Kord's Child the ability to use his Channel Divinity for the spell "Caught in the Storm".  For all intents and purposes the "Zephyr - Awoken" monster statistics are to be used.

From now on Zephyr shares his health pool with Kord's Child. Upon taking damage health is always deducted evenly, with odd damage numbers on Zephyr first. As long as Kord's child is on the same plane, both can communicate with each other and - spending an action - perceive through each other's eyes and ears. If Zephyr dies and cannot be brought back to life until the next sunrise, the connection breaks and Kord's child will inevitably shatter in his soul.

The hammer Zephyr itself will transfer its power to Kord's child. It will become a magical +2 hammer with a slight bluish glow on its exterior. Each (even unsuccessful) attack increases the damage by 1 thunder damage (starting at 1) until it reaches 6. On the last discharge (the 6th attack), one of the following effects is triggered instead of the accumulated damage. Roll a D4

D4 Roll at 6 charges	Effect
1-3	cast the spell Thunderwave at level 4
4	cast the spell Destructive Wave

 

Caught in the Storm.
As an action you can use your channel divinity to activate "caught in the storm". This effect does not require concentration.
Zephyr - heart of restrained thunder - will create a temporal rift between the planes and allow the divine realm of Kord to permeate the current plane of Zephyr's bearer. This effect lasts until the end of your next turn. In a 10 ft radius around Zephyr (the Griffon) and it's bearer all celestial, fiend, fey, elementals and undead take 2d8 thunder damage upon entering.

At the end of your turn designate any number of creatures within the effect's area. These creatures benefit from the Sanctuary spell's effect.

Proficiency with a Warhammer allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, any creature with good or chaotic alignment, Versatile, Push