Tales of the Twilight Coast
When a player, NPC, or monster rolls a natural “1” on an attack it will be an automatic miss regardless of adjustments to the attack roll per the PHB. In addition, the DM may determine a special effect appropriate to the specific situation such as a dropped weapon, falling prone, or loosing the next round of attacks. The occurrence of and severity of said effect is at the discretion of the DM.
Additionally, in the rare case that a player, NPC, or monster rolls two “1’s” on an attack roll when attacking with advantage or disadvantage the DM will determine a severe consequence appropriate to the specific situation up to and including a broken weapon, injuring an ally, or injuring self.