If you own the game (and you've been able to add it for free a month or two ago) , you can pick up some free DLC for Borderlands 2. This involves Commander Lilith & the Fight for Sanctuary DLC for Borderlands 2 starting September 1st and lasting until October 2nd 2020.

Borderlands 2: Commander Lilith & the Fight for Sanctuary – Sanctuary is under siege, the Vault Map has been stolen, and a toxic gas is poisoning Pandora. Battle new bosses, explore new areas, collect new loot (including an entirely new tier beyond Legendary), and join up with Lilith and the Crimson Raiders to take on a deranged villain hell-bent on ruling the planet.

You need to have base game Borderlands 2 to play this, but as you can remember, it was already given away by Epic a while ago. 

Head over to Epic Games Store or navigate to the game in their launcher before October 2nd 2020, add the game to your cart and checkout. Simple as that. After that, the DLC is yours to keep, and play, forever.