Winter was coming on and the hills threatened the surrounding villages with destruction, for heavy rain would not only wash away the soil but would cause serious floods as well.
これを訳すと、
冬が到来して、豪雨は土石流だけだなく、大洪水ももたらす可能性があり、この丘は近隣の村に破壊の脅威を与えた。
threaten 脅かす
文末に as well を付けて、
おまけに、そのうえ
例).I am young, and beautiful as well.
私は若い、しかも美人です。