Complex sentence: His weakness is that his undefeated title has given him a huge ego, that causes him to underestimate his enemies, leading to his untimely defeat by Rupert, Bill, Algae.
Split into simpler sentences: His weakness is that his undefeated title has given him a huge ego. That causes him to underestimate his enemies which leads to his untimely defeat by Rupert, Bill, Algae.