This weapon set borrows a bit of the ranger's dual-wielded weapons concepts, applying extra damage over multiple hits. Like hammering in a nail, hammering in this stitching awl deals extra damage, for which you benefit if you have a bonus action attack! 1d6 per hit seems strong, but considering these are d4/d6 weapons (and you'd have to hit with the d4 weapon first), it's comparable to a regular martial weapon.
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The mallet and stitching awl are ubiquitous among cobblers who punch holes in their leather for lacing. While nothing special on their own, they can be especially deadly in the right hands, particularly in those of master artisans.
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Skull Piercer
Weapons (light hammer and dagger), rare (requires attunement by a character proficient with cobbler's tools)
The mallet (light hammer) and awl (dagger) come in a pair and you attune to them as a group. You can use them as cobbler's tools for punching holes in leather and while attuned, you also double your proficiency bonus to ability checks made using cobbler's tools. Creatures unaware of the weapons' properties initially perceive them as simple tools and can use an action to recognize their true purpose by succeeding on a DC 15 Intelligence (Arcana or Investigation) check.
Unassuming Tools. You have a +1 bonus to attack and damage rolls made with these magic weapons. When you hit a surprised target with either weapon, you can roll twice as many dice for their weapon damage.
Awl Driver. Whenever you use the mallet to hit a target you've already hit with the awl since the start of this turn, you deal an extra 1d6 piercing damage.