The Smog is a Legendary Hyperion SMG that is available in Borderlands 3. You can get the weapon as a random drop from Scourge the Invincible Martyr in the Takedown at the Guardian Breach. You only gain access to the Takedown at the Guardian Breach after you have finished the main story. Once the campaign is finished, you can go and visit Tannis in the Med Bay on Sanctuary III, and interact with the blue crystal on the shelf directly across the from the main door.
While you can jump straight from the campaign into the Takedown, it is advisable to level up first and farm some better weapons. Scourge is the effective raid boss of the Takedown, so you will want to be well-armed to take him out. The Smog can drop when Scourge is killed, but this is a random loot drop. Like all Legendary weapons in the game, drop chances get higher as the difficulty goes up, so people playing True Vault Hunter Mode at higher Mayhem levels will have the best chance to get it.
The Smog can drop with any of the following elements, or with no element, making it a solid option for builds as you can simply switch the weapon for a new element type when you need to.
The main perk of the Smog is that while you are aiming down sites, and the Hyperion shield is at max capacity, you will get a bonus to the damage. This pretty much doubles your damage output, making the first few shots from it hugely effective, although this stops when the shield takes damage. The perk kicks back in again after the weapon shield is fully powered up.