A Torgue pearlescent rocket launcher.
Red text: Here comes the Sun!
It's a reference to a Red alert 2 Mental omega's infantry, the Eradicator.
It shoots a horizontally waving beam, which deals radiation damage, and you deal continuous radiation damage to nearby enemies, while holding the weapon. The weapon is black coloured with some orange parts, and a radiation icon on it. (And if it's possible, when you shoot, it has a sound like in the original red alert 2 game.)
And yeah, the radiation would be a new element! It takes a low amount of damage/tick, but lasts until the enemy dies, and has no damage buff (or other) effect, and no damage multipliers on flesh/shield/armor. And its color is orange.