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Chaos Islands are terrain generation features added by the Draconic Evolution mod. The islands generate in the empty space of the End at any set of coordinates which is divisible by 10000 (by default, this can be changed in the config), such as (x:0, z:10000). They are made up out of a solid center made up out of End Stone. Moving outwards, the island has more and more empty space in between the blocks and also contains Obsidian blocks. They also generate more upward and downward on the y-axis, forming a bowl shape both at the top and bottom of the island. The entire island is surrounded by a ring-shaped disc of End Stone.
Aside from the Obsidian blocks, a lot of terrain generation from the main island of the End also occurs on Chaos Islands, things like Oblivion Hives and Ender Lilies can also be found there. The main features of the island are an inner ring of Obsidian pillars, such as on the main island, as well as an outer ring of Obsidian pillars which have cages around the top, made up out of Nether Brick Slabs and Iron Bars.
Every island will have a Chaos Guardian boss mob spawning on it, which can be killed by the player to obtain Dragon Hearts, also, beware the previously mentioned Chaos Guardian for it can kill you in Creative mode.
(old info: The bigger the coordinates of the island are, the stronger the Chaos Guardian is. The Chaos Guardian's health is 1/10 of the set of coordinates. The larger number is used, so at x=20000, z=10000, it will take the 20000 and the Chaos Guardian will have 2000 hp(10 Ender Dragons or 1000 hearts). For Nov.2018 all default Chaos Guardians have 2000 hp.)
The center of the island will also contain a Chaos Shard. This Shard can be removed once the Chaos Guardian is killed. Its location is shown by seemingly endless amounts of lightning on the surface. Once you hear a strange, eerie type of sound, you know you are there next to it. Once removed, the shard will trigger a delayed explosion of the island. After you take the shard, the camera will start to shake and will keep doing so until either a) you leave the now-doomed island and don't return, or b) the island blows up. When the island blows up, it will take about 1000 points of shields off Draconic armor, or straight up kill you.