To attune: The wielder must wrap the haft his hand with a branch of livewood, and hold it for 24 hours without letting it go. The fire within it will burn, and the wielder will take 1d6 of fire damage every hour.  

Onatar’s hammer works wonders and disasters, the god of the forge using it to craft ground-breaking marvels and dangerous inventions in equal measure. When Onatar does permit a mortal to use the hammer, it’s usually so they might bring an important work into being, wreck a force of destruction, or forge something remarkable somewhere the god isn’t welcome.

Hammer of the Forge. This magic warhammer grants a +2 bonus to attack and damage rolls made with it. When you hit with an attack using it, the target takes an extra 2d10 fire damage.

Blessing of the Forge. If you are a worshiper of Onatar, you gain all the following benefits:

Level 1-5: Once per day, you can use the heat metal spell without using a spell slot. It is always considered a prepared spell for you. 
Level 5-9: Once per day for an amount of rounds equivalent to your character level, you may change your weapon's material to be a bane of a specified creature type (Aberration, Undead, Beast, Monstrosity, Humanoid). 
Level 10-14: Once per day, you can use the shatter spell without using a spell slot. It is always considered a prepared spell for you. 
Level 15-19: You gain immunity to fire.
Level 20: Fire damage will heal you. When hit with a flaming weapon or a fire typed damage, divide it by half, and heal up to your max HP. 

 Repair, Resist. You may cast the cantrip mending and , resistance at will. 

Reforged. While holding the hammer, you have resistance to fire damage and are immune to exhaustion. Additionally, you have proficiency with smith’s tools and have advantage on all ability checks made using them. If you already have proficiency with smith’s tools, you gain expertise.

Spells. While the hammer is on your person, you can use an action to cast one of the following spells (save DC 18): animate objects, heat metal, fabricate, magic weapon, shatter. Once you use the hammer to cast a spell, the spell can’t be cast again from it until the next dusk.

Destroying the Hammer. To destroy the hammer, it must be taken to the realm of Risia, ithe Plane of Ice. There, it must be quenched in the Sea of Marid. The heat of the hammer cools and its fire extinguishes after one month. Once fully cooled, the hammer reverts to a base hammer of its original materials. 

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

Notes: Immunity: Exhaustion, Resistance: Fire, Bonus: Magic, Damage: Fire, Expertise: Smith's Tools, worshipper of Onatar., Versatile, Push