Dark Thanos was a doppelganger created by the Magus
prior to the Infinity War
. Unlike all his other doppelgangers, Dark Thanos has the exact appearance and mind of the original. Because of his mind, the Magus made him his assistant because of his immense intelligence and scheming abilities. But the Magus succeeded too well in recreating Thanos, and Dark Thanos secretly plotted to take power from the Magus when the time was right.
His original purpose was to have the heroes involved to believe Thanos
was behind the plot all along. This was successful until Galactus
captured them as they battled and read their minds, thus explaining the true situation to all.
When the time came, he had to face the true Thanos in battle. Dark Thanos exceeded Thanos in sheer power, being augmented by Magus specifically to destroy Thanos. He also had not gone through the life changing experience of becoming omnipotent as the real Thanos, making him as powerful and determined as the Thanos of old. The Magus allowed them to battle and told them only one of them could join him. They temporarily allied with each other but soon realized only one could survive. Though neither Thanos or Dark Thanos wished to join the Magus as a servant, they knew there could only be one.
They battled each other furiously and Dark Thanos had the clear advantage. The true Thanos overcame his self doubt and defeated Dark Thanos. He transformed Dark Thanos into a butterfly and ate it, absorbing all his strengths and the knowledge of the Magus and his insights into Thanos' mind. The true Thanos remained and was finally complete once again.