Demetrius and Chiron are Tamora's wicked sons. They kill Bassianus and then viciously rape and mutilate Lavinia in the woods after Aaron and Tamora encourage them to do so.
Demetrius and Chiron aren't very bright – they can't even come up with their own evil plots and they're easily duped by Titus. These brothers are pretty much synonymous with savage cruelty. After assaulting Lavinia, they taunt her mercilessly. Check out how they're portrayed in director Julie Taymor's film Titus. (Warning, this film clip is graphic and may offend some viewers.)
But don't worry, these boys get their just desserts when Titus kills them and bakes them into a pie, which he then serves to their mom.