4 A long-lost brother, who was presumed dead, was really alive all the time and wichedly plotting to bring about the hero's downfall. And so on.

who 为关系代词, 引导非限制性定语从句, dead作从句主语的补足语

背熟：who was presumed dead, was really alive all the time and wickedly plotting to bring about the hero's downfall.

presume: 假定 (根据过去的经验和感觉作出决断)  未证实前假定是真的

assume : 武断的，把尚未证实的作为依据，与事实不相符合

Eg: Let us assume that your words are fact

suppose : 最普通的用词，猜想、陈述自己的看法

Eg: I suppose that it was my fault.

conspire: 众多人合谋做重大的犯法勾当，犹指反叛，不用于修饰单个人

plot 密谋

bring about   造成, 导致,引起

Eg:

Gambling has brought about his ruin.

downfall: sudden & complete  毁灭性结局新概念英语青少版