EGO
Universe: The Marvel Cinematic Universe.
Profile: Ego is a Celestial: a primordial and extremely powerful being, as well as the biological father of Peter Quill AKA Star-Lord. He developed the ability to manipulate molecules and matter, which led him to construct a protective core around himself. Over the course of many years, he continued to build planetary layers over the core which evolved into a planet itself - and that's where this arena is.
Location: Ego has a palace of sorts, which can be seen at
1:20 in this video. Each team will start two miles away from it on opposite sides, making them four miles apart from each other in total. The landscape they're on is luscious, beautiful and colourful, but it's also hilly and there are lots of tall trees (so line of sight certainly won't be possible for all but the tallest characters).
NPCs: Ego's humanoid self will be in the palace - and he doesn't take too kindly to visitors (especially if they directly affect his humanoid self).