"fell into a very deep sleep" or "slept deeply." It means that he was very much asleep.
## deep and terrifying darkness ##
"extreme darkness that that terrified him"
## overwhelmed him ##
"surrounded him"
## strangers ##
"aliens" or "foreigners"
## and will be enslaved and oppressed ##
This can also be translated with an active clause: "and the owners of that land will enslave your descendants and oppress them." (See: [[rc://en/ta/man/translate/figs-activepassive]])