Thanos is a villain from the Marvel Universe who simply cares about conquest of many worlds. Both the High Council and the Devils have neither made an effort to recruit him since the Mad Titan has a tendency to shift between good and evil.
Somehow in his early life, he developed a morbid obsession with death, which attracted the attention of the Devils. After gaining powers surpassing that of his own kind, he ended up stealing a starship to recruit soldiers for his evil army as part of his plan. After conquering his home world and becoming his ruler, Thanos set his sights on Earth to conquer it.
Once, he met the personification of Death and fell in love with her. To make himself worthy of Death's love, he plotted to conquer many worlds, including Earth itself. Since then, he is currently in some battles with the Avengers and other superheroes.
The Devils though are still wary of him since he likes scamming beings higher than himself such as Mephisto to gain more power. After all, making Thanos aware of the Devils and the Council would be an open invitation for him to overthrow them from within, if not play them against one another for his own benefit. He is a very powerful piece indeed; but perhaps powerful enough to justify leaving off the board. If he discovers their existence, and invites himself onto the board, who knows the outcome of that game...
Other Facts Edit
He is the maker of the Infinity Gauntlet.