Thanos is a Titan from a planet called Titan. He was an outcast from the rest of the Titans because he was deformed,
discolored, and had genius level intellect. Nevertheless, he loved his people. When their planet was on the brink of extinction, he offered up a solution: kill half of everyone on the planet, completely randomly, so the other half could thrive. Of course, he was deemed a Mad Man and nobody believed his plan would work. When the inevitable happened and everyone died on his planet (aside from him), he decided that his idea was correct. So he started a war campaign and killed half of all life on every planet. Eventually, he got to Earth, fought the Avengers, and got the Infinity Stones. When he got these gems, he snapped his fingers, and half of life died on every planet in the Universe. This included Eluryh. King Harkinian was outraged, so he slapped Thanos until he brought everyone on Eluryh back to life.