Maveth
Maveth is a planet located in the Galactic Rim. It is accessibly by other means, but most travellers to the planet reach it via Kree teleporters, sometimes called Monoliths.
Thousands of years ago, it was home to an advanced civilization. This was destroyed when the Kree began using it as a penal colony. This practise was continued by the early Inhumans, who exiled King Alveus to the planet. Over time the native inhabitants and exiled Inhumans died out, leaving only Alveus, whose powers rendered him functionally immortal.
In 2001, NASA launched an expidition through one of the surviving Monoliths. The expidition team found they were unable to return, and all but one of the expidition astronauts were killed by the harsh environment, or chose to take their own lives rather than remain trapped on Maveth. Will Daniels apparently survived, although it was later discovered that he was in fact being used as a host body by Alveus.
S.H.I.E.L.D. scientist Jemma Simmons was transported to Maveth when a Monolith activated while she was studying it. She was eventually rescued by Leo Fitz. Believing Will Daniels to still be alive, S.H.I.E.L.D. launched an expidition to Maveth to retrieve him. Alveus was able to use the body of Hydra agent Grant Ward to escape the planet. Since then, it has been uninhabited.