The Sunstorm is the boss of the Eden Dimension. You are required to kill this boss to be able to advance to The Wildwood. The boss spawns in a small, shrine-like structure with an empty yellow spawner in the centre.
Small, shrine-like structures can be found randomly in the Eden dimension. Inside of these structures is an empty yellow spawner. When the player right clicks on it with an Eden Chunk, a message will appear in the chat that says "Sunstorm will spawn in 5 seconds," after which the Sunstorm will spawn. The Sunstorm itself is a tall, yellow/orange boss that shoots yellow light beams at you which do a moderate amount of damage and obscure your vision.
The best tactics for defeating the Sunstorm are:
Rushing - A more risky, melee based strategy that should only be attempted in full enchanted armour, at least Diamond or Eden, and an Eden blade. The boss also has a slightly less damaging melee attack that it only uses when the player is close.
Ranged - A slower, but ultimately safer strategy involving the use of a bow and cover. The best way to use this tactic is to knock the boss into a pool of water that it cant escape from, and then firing arrows at it and hiding behind cover when it makes its attack, which travels rather slowly so you should have time to shoot a few arrows and take cover.