Tridax was a Trister who spawned an Accelerak. They were exceptionally powerful for Bakugan of their species, and were quickly inspired to rule the world through brawling. They brawled the president between part 2 and 3, and were thrown into a nuclear lab when they proved too powerful. They were combined and mutated, forming Tridax.
Tridax can spew water out of his giant forelegs, and fire sonic waves through them out of the circles on his body. The rudders on his chest can send torrents, and his stomps create tsunamis. However, if the water he supplies from his forelegs is evaporated and the forelegs cannot produce water (for any reason), he has no power aside from his physical strength. The fins on his body allow him to swim flawlessly despite his weight and body shape.
Tridax joined Necronoid because he wanted to rule the world. His intelligence suffered due to having two nearly opposite personalities combined. That makes him very obedient to Necronoid, and something of a hound.