In the reaction:
Mg + Cl2 --> Mg2+ + 2Cl-
The reduced substance is the substance that the gained electron/s.
Looking at the substances:
Mg has a neutral charge, after the reaction, it gained a charge of 2+. This means, it lost 2 electrons.
Cl2 has a neutral charge, after the reaction, it gained a charge of -1. This means it gained 2 electrons.
Therefore, the reduced substance is Cl2.
