Titus Livius (Livy), The History of Rome, Book 2
Benjamin Oliver Foster, Ph.D., Ed.

20. When the Romans were now beginning to give way in that part of the field, Marcus Valerius, Publicola's brother, espied the young Tarquinius, who was boldly inviting attack in the front rank of the exiles. [2] Valerius found in his brother's glory an additional incentive, and resolving that the family which had the honour of expelling the tyrants should also gain the credit for their death, he dug his spurs into his charger and rode at Tarquinius with levelled spear. [3] Tarquinius drew back within the company of his followers to avoid his desperate antagonist. Valerius was plunging blindly into the exiles' line when one of them attacked him in the flank and ran him through the body. But the rider's wound did not check the career of his horse, and the dying Roman came down in a heap upon the ground with his arms upon him. [4] When the dictator Postumius perceived that so brave a soldier had fallen, that the exiles [5] were advancing boldly at the double, and that his troops were checked and were giving [p. 283]ground, he issued orders to his own cohort, a picked¹ body of men which he kept about his person as a guard, that if they saw any Roman running away they should treat him as an enemy. [6] Being thus between two dangers, the Romans faced about to meet the foe, and the battle-line was formed again. The cohort of the dictator then entered the engagement for the first time. [7] With fresh strength and spirit they attacked the weary exiles and cut them to pieces. Then began another combat between leaders. The Latin general, perceiving that the cohort of the exiles was nearly cut off by the Roman dictator, took a few companies of his reserves and hurried them to the front. [8] As they came marching up, Titus Herminius, the lieutenant, caught sight of them, and in their midst, conspicuous in dress and accoutrements, he saw and recognized Mamilius. [9] Whereupon he hurled himself upon the enemy's commander with so much more violence than the master of the horse had done a little before, that not only did he pierce Mamilius through the side and slay him with a single lunge, but in the act of stripping the body of his antagonist he was himself struck by a hostile javelin, and after being borne off in the moment of victory to the Roman camp, expired just as they began to dress his wound. [10] The dictator then dashed up to the knights and besought them, since the foot-soldiers were exhausted, to dismount and enter the fight. They obeyed: they leaped down from their horses, hastened to the front, and covered the front-rankers with their shields. [11] It restored at once the courage of the foot to see the young nobles on even terms with themselves and sharing in the danger. Then at last the [p. 285]Latins received a check, and their battle-line was² forced to yield. [12] The knights had their horses brought up that they might be able to pursue the enemy, and they were followed by the infantry. Then the dictator, neglecting no help, divine or human, is said to have vowed a temple to Castor, and to have promised rewards to the soldiers who should be first and second to enter the camp of the enemy; [13] and so great was the ardour of the Romans, that with a single rush they routed their opponents and took their camp. Such was the battle at Lake Regillus. The dictator and his master of the horse returned to the City and triumphed.