What is mortal sin? Mortal sin is a grievous offense against the law of God.
Flee from sins as from the face of a serpent; for if thou comest near them, they will take hold of thee. (Ecclesiasticus 21:2)
Why is this sin called mortal? This sin is called mortal, or deadly, because it deprives the sinner of sanctifying grace, the supernatural life of the soul.
Before man is life and death, good and evil; that which he shall choose shall be given him. (Ecclesiasticus 15:18)
Besides depriving the sinner of sanctifying grace, what else does mortal sin do to the soul? Besides depriving the sinner of sanctifying grace, mortal sin makes the soul an enemy of God, takes away the merit of all its good actions, deprives it of the right to everlasting happiness in heaven, and makes it deserving of everlasting punishment in hell.
For the wages of sin is death; but the grace of God, life everlasting in Christ Jesus our Lord. (Romans 6:23)
Further reading: CCC 1854-1856