Arad's options had long since ended. He can't fight the twins at close range since they are older, stronger, and faster. He can't challenge them at range with arrows, bolts, and stones since mundane objects have no chance of harming them. Even the enchanted ones would be useless. Those two are Mana Drakainas- they'll just disable the enchantment since arrows, bolts, and stones aren't connected to Arad at range. Arad can try to keep the link in his domain. But who is he kidding? He has no control over the Mana in the air when two Mana Drakainas are around.
Magic was also out of the question since it would be useless against them due to their very nature. That is why all of Arad's options had run dry, leaving him only his blood, which was still a part of him even when separated from him.
Blood magic was directly linked to him since he is a Pricolici, and the link was even stronger than what it would be for a normal vampire. That is why his blood was his only and best-ranged option.