From among all those who have died. This expression describes all dead people together in the underworld. To come back from among them speaks of becoming alive again.
The person who sits "at the right hand" of a king sits at his right side and rules with all the authority of the king at whose right hand or side he sits. This is a metonym of location that represents the authority that the person in that location possesses. Alternate translation: "gave him all authority to rule from heaven" (See: [[rc://en/ta/man/translate/figs-metonymy]])