Zoldeo is a member of the Grimoire Heart Guild and one of the Seven Kin of Purgatory through possession of Capricorns body. Zoldeo gained the Capricorns key after Layla Heartfilia retired. He was expected to hand it over to Lucy when she was ready to use Magic but he became twisted and corrupted and swallowed up by the darkness. At one point he broke a taboo of the Celestial Spirits by using his Human Subordination Magic and somehow ended up possessing Capricorns body. Its because of this that Capricorn was able to stay in the human world for 17 years. At some point in time he was found by Master Hades and joined Grimoire Heart becoming one of the Seven Kin of Purgatory. Source: Fairy Tail Wikia