Tyrion parries the first enemy hit against him per Close Combat phase and it has no effect. In addition, assuming Tyrion is still alive after all other attacks have been made, he may make a single extra attack of his own back against the model that he parried. This attack is in addition to those normally allowed to Tyrion and follows all of the rules for Tyrion's attacks. This has no effect on special attacks which do not use the model's Attacks characteristic (such as a Night Goblin Fanatic or a Giant).