From the Catechism of St. Pius X, "The Main Kinds of Sin,"
Q: What injury does mortal sin do the soul?
A: (1) Mortal sin deprives the soul of grace and of the friendship of God;
(2) It makes it lose Heaven;
(3) It deprives it of merits already acquired, and renders it incapable of acquiring new merits;
(4) It makes it the slave of the devil;
(5) It makes it deserve hell as well as the chastisements of this life.
Q: Besides grave matter, what is required to constitute a mortal sin?
A: To constitute a mortal sin, besides grave matter there is also required full consciousness of the gravity of the matter, along with the deliberate will to commit the sin.