Fear and Horror

When adventurers confront threats they have no hope of overcoming, you can call for them to make a Wisdom saving throw. Set the DC according to the circumstances. A character who fails the save becomes frightened for 1 minute. The character can repeat the saving throw at the end of each of his or her turns, ending the effect on the character on a successful save.
Horror involves more than simple fright. It entails revulsion and anguish. Often it arises when adventurers see something completely contrary to the common understanding of what can and should occur in the world, or upon the realization of a dreadful truth.
In such a situation, you can call on characters to make a Charisma saving throw to resist the horror. Set the DC based on the magnitude of the horrific circumstances. On a failed save, a character gains a short-term or long-term form of madness that you choose or determine randomly, as found here.