en_tn/gen/18/18.md

all the nations of the earth will be blessed in him

This can be stated in active form. AT: "I will bless all the nations of the earth through Abraham" (See: rc://en/ta/man/translate/figs-activepassive)

will be blessed in him

"will be blessed because of Abraham" or "will be blessed because I have blessed Abraham." For translating "in him" see how you translated "through you" in Genesis 12:3.