Instead of the shock cave mission mentioned by Dibbs93, you can also complete the mission "The Legend of Moe and Marley". You can get the mission from the Fyrestone Bounty Board after Sledge: The Mine key is completed. For the next main story mission, you need to go through Arid Hills to get to Sledges Safehouse anyways, which is the location of Moe and Marley.
They are variants of Badass Fire and Shock Skags and can be a bit of a pain to beat. I recommend doing the main quest first and try to kill them on the way back. I was able to kill them at level 15 with a blue Tediore Shotgun and a blue Maliwan SMG. Like with all Skags try to shoot their mouth when open for crit damage and stay away from Marleys shock blasts, or your shield will deplete quickly.
You'll get a fire artifact as reward, just waiting in your inventory to be equipped.