Obsidian Dagger (5e Equipment)
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Weapon (dagger), unique
The ultimate stabbing device. Simple, but effective.
Violent Glass. This beautiful, gem-encrusted dagger is made entirely of obsidian (with many, different colored gems embedded in it for flavor and appearance), yet always seems to be perfectly weighted for the hand that wields it, changing size and shape to the perfect ideal of what any given wielder would prefer most with their dagger. This dagger has a +5 bonus to attack and damage rolls, and transforms when first picked up by a wielder into the perfect dagger for them. The wielder has no control over this transformation, but it remains consistent with their opinion of the absolute highest quality weapons.
DM Note: Fun fact for you, apparently up to a +4 weapon can exist in D&D 5e according to WOTC, though the trend of magic items with properties suggest giving it no other properties if you ever make one (legendary weapons with other properties never have a +4 bonus, capping out at +3 instead). This is why I made it one of the simplest, and debatably weakest weapons in the game, but it'll still be highly valued by any rogue or Dexterity paladin for it's sheer accuracy alone. I couldn't find official material that gives anything higher than +4 however, so that's why I made it unique rarity, at least until I find the evidence to change it.