The entire. Darned. Mod. It's less of an addition and more of an expansion on a rejected concept (the not-included, unnamed Sky Dimension that the End replaced), taking the original idea and blowing it out into something far better. If only these guys had been on the original Minecraft team...
The battle against the Sun Spirit is pretty epic.
You can break more than one Slider out of his prison and get them all to fight you at once. The ensuing battle is epic, and you are awesome if you win.
Two Tempests can kill a Wither. This troper tested it, and it's true.