Spellneedle

Weapon (dagger or spear), very rare (requires attunement)

This mithral dagger has a hollow, telescoping handle. While holding the weapon, you can speak its command word using a bonus action to cause it to transform into a spear or back into a dagger, extending or retracting the handle respectively. Regardless of the weapon's form, it has the finesse property, and you are proficient with it. If you're a spellcaster, you can use your spellcasting ability for its attack and damage rolls, instead of Strength or Dexterity.

You gain a +2 bonus to attack and damage rolls made with this magic weapon.

While holding the weapon, you can use your reaction whenever a creature you can see within 30 feet of you casts a spell, allowing you to collect one of the arcane threads used to produce the spell. Only you can see this thread, which remains woven through the spellneedle until the end of your next turn or until you use the thread. The weapon can only have one thread at a time.

While holding the threaded weapon, you can expend the thread to gain one of the following benefits.

Stitch Burst. If you make an attack with the weapon, you can expend the thread (no action required) to deal an extra 1d10 force damage to the target on a hit; each other creature within 5 feet of the target (other than you) must succeed on a DC 16 Dexterity saving throw or take the same amount of force damage. If you made a ranged attack with the weapon, it's magically pulled back to your hand immediately after the attack.

Unspool Spell. If you're forced to make a saving throw against a spell, you can expend the thread (no action required) to roll a d10 and add the number rolled to the saving throw. In addition, any damage you take from the spell is reduced by an amount equal to the number rolled on the d10.

The spell came undone like a threadbare rag in the wizard's hand. Facing him, a mage hunter spun their dagger with practiced mastery and gave only a smirk. They flicked their wrist, telescoping the dagger's handle to the floor to transform it into a shimmering spear.

The hunter leveled the spear at their prey and said, "Salazar, it's in your best interest to surrender now."