It was believed by some that the power of persons whose heads were taken was added to that already possessed by the successful warrior or his village.

1. that 是个“替代词”，替代前文的 power（力量）。

2. 划线部分是过去分词短语作 that 的定语。

3. 这句话中，that the power of persons whose heads were taken was added to that already possessed by the successful warrior or his village 是主语从句。在这个从句中，划线部分是 persons 的定语从句。由于是被动语态，原来的宾语是 the power 变成了主语。