You have a +1 bonus to attack and damage rolls made with this magic weapon. 

On a successful hit you make another attack roll on another creature within 10ft of the attacked creature adding +1d4 Bludgeoning to damage and making another attack roll on a different creature within 10ft of the new attacked creature and so on and so forth. After 5 successful hits you add 1d6 fire damage to the next attack and the you add 1d6 fire additional as well as the +1d4 Bludgeoning. The boomerang will Cap at 10 total creatures hit making it 1 d4+(DEX)+9d4+5d6(fire)

On the first miss made the Boomerang returns to the owner, if a miss was made on the first attack they just catch the boomerang but if the boomerang hit a Creature then the user must make a DC5 DEX saving through or take the same damage as the last creature hit, every additional hit made by the boomerang before a miss adds +2 to the DC. If the cap of 10 Creatures is hit then the DC will be DC25 DEX saving Through

The boomerang is a ranged weapon, and any creature proficient with the javelin is also proficient with this weapon.

On a miss, a boomerang returns to the thrower's hand.

Notes: Bonus: Magic, Damage: Bludgeoning, Damage: Fire, Damage, Combat, Range