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Rat Bat
Rat Bat
Rare
1d8+1 Bludgeoning
Rat Catcher: Attack Rolls against beasts have Advantage.
Weapon Enchantment +1
Proficiency with this weapon type unlocks:
A club with nails hammered through it. Classic.Greatclubs Two-Handed
Location - Rat Bat
Located in 
The Waning Moon. Resting against the bar.
Discussion
The Waning Moon X: -224 Y: -78: Can be found leaning against a wall, near Thisobald Thorm.
It is not listed anywhere in-game, but the Rat Bat does an extra 1d6 Piercing DRS damage against any target.
Unlike other greatclubs, this weapon does not have the Tenacity feature.