Weapon (longbow), uncommon
You have a +1 bonus to attack and damage rolls made with this magic weapon.
Proficiency with a Longbow allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Slow. If you hit a creature with this weapon and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.
Notes: Bonus: Magic, Damage, Combat, Ammunition, Heavy, Range, Two-Handed, Slow
How much would this cost
Since it’s an uncommon magic item, 500gp may be pretty sufficient for it.
I know that I’ve read somewhere that regular arrows used with a magical bow are counted as magical attacks for the purposes of resistances and immunities but I can’t figure out where I read that. Anyone have a citation for me?
UPDATE: Nevermind. Found it here