Thanos is a powerful alien, a warlord who rules over a dead dimension. He allies himself with the exiled Loki Laufeyson, providing him with the deadly army of the Chitauri, who will help him conquering Earth, in exchange for the Tesseract, a powerful energy source.
When Loki arrived on Earth, Thanos is contacted by him, and the Other warns the God of Mischief of the price of failure and reminding him to not question the will of the master, declaring that if he fails in defeating the Earthlings, Thanos will make him know a new kind of suffering.
When eventually Loki is defeated, The Other complains that the Earth's inhabitants are not as weak as they were supposed to be and declares that challenging the Earth's heroes would be to court death. To this, Thanos simply smiles.