Terramorphous the Invincible is a Raid Boss in Borderlands 2. They are located in the Terramorphous Peak region on the Planet Pandora which you can access through the Thousands Cuts Region.
You encounter Terramorphous during and after the optional quest “You. Will. Die. (Seriously.)” which comes available after you have beaten the main story campaign. It’s obtained from Patricia Tannis in Sanctuary.
You can farm Terramorphous for the Blood of Terramorphous, Hide of Terramorphous, Slayer of Terramorphous, and the Pitchfork as it is the dedicated loot source for this item. Terramorphous’ unique and legendary items have a 2.86% drop chance but the Class Mod has a 16.7% drop chance.