Enough games, Thanos. You will tell me where she is...or I will level this world molecule by molecule.-Walker
Walker is a death god native to the Marvel universe and a former rival to Thanos turned enemy of Death itself after the entity rejected his "gift". He is arguably one of the most powerful villains in the Marvel universe despite his relative obscurity. He also appears in the Captain Marvel series starring Genis-Vell and Rick Jones.
The enigmatic Death-God hailed from a far distant Galaxy. As Death hid herself within Marlo, Thanos sought the means to dispose of Walker. He later recruited the bestial being known as Mangog in what was yet another test. As Thanos enlisted the aid of Thor, Jones summon Genis, the son of the original Captain Marvel, with whom he was bonded. The trio revealed the truth of Marlo's rebirth, and the severity of the threat they all faced, Walker alongside Thanos. Walker overcame them, and continued to torture Rick Jones, warping time around him and aging him many years as a result.
It was as Thanos well knew, Death was not only a taker of life, but also Marlo and attacked Walker, promising to cause the former Death-God an eternity of torment. Death restored Marlo's life, and promptly returned to her realm.