Titus Livius (Livy), The History of Rome, Book 5
Rev. Canon Roberts, Ed.

15. During¹ this period many portents were announced, but as they rested on the testimony of single individuals, and there were no soothsayers to consult as to how to expiate them, owing to the hostile attitude of the Etruscans, these reports were generally disbelieved and disregarded.

One incident, however, caused universal anxiety. [2] The Alban Lake rose to an unusual height, without any rainfall or other cause which could prevent the phenomenon from appearing supernatural. Envoys were sent to the oracle of Delphi to ascertain why the gods sent the portent. [3] But an explanation was afforded nearer at hand. [4] An aged Veientine was impelled by destiny to announce, amidst the jeers of the Roman and Etruscan outposts, in prophetic strain, that the Romans would never get possession of Veii until the water had been drawn off from the Alban Lake. This was at first treated as a wild utterance, but afterwards it began to be talked about. [5] Owing to the length of the war, there were frequent conversations between the troops on both sides, and a Roman on outpost duty asked one of the townsmen who was nearest to him who the man was who was throwing out such dark hints about the Alban Lake. [6] When he heard that he was a soothsayer, being himself a man not devoid of religious fears, he invited the prophet to an interview on the pretext of wishing to consult him, if he had time, about a portent which demanded his own personal expiation. [7] When the two had gone some distance from their respective lines, unarmed, apprehending no danger, the Roman, a young man of immense strength, seized the feeble old man in the sight of all, and in spite of the outcry of the Etruscans, carried him off to his own side. [8] He was brought before the commander-in-chief and then sent to the senate in Rome. In reply to inquiries as to what he wanted people to understand by his remark about the Alban Lake, he said that the gods must certainly have been wroth [9??] with the people of Veii on the day when they inspired him with the resolve to disclose the ruin which the Fates had prepared for his native city. [10] What he had then predicted under divine inspiration he could not now recall or unsay, and perhaps he would incur as much guilt by keeping silence about things which it was the will of heaven should be revealed as by uttering what ought to be concealed. [11] It stood recorded in the Books of Fate, and had been handed down by the occult science of the Etruscans, that whenever the water of the Alban Lake overflowed and the Romans drew it off in the appointed way, the victory over the Veientines would be granted them; until that happened the gods would not desert the walls of Veii. Then he explained the prescribed mode of drawing off the water. [12] The senate, however, did not regard their informant as sufficiently trustworthy in a matter of such importance, and determined to wait for the return of their embassy with the oracular reply of the Pythian god.