[Bernard was called to Guienne], where William, the powerful and haughty duke of that province, violently persecuted those who adhered to the true pope, and had on that account expelled the bishops of Poitiers and Limoges. Gerard, bishop of Angouleme, an abetter of the schism, encouraged him in these excesses. This William ... was a prince of high birth, immense wealth, a gigantic stature and strength of body, and extraordinary abilities in worldly affairs; but was in his youth impious, haughty, and impatient of the least control. ...
... The duke listened to [Bernard] with great respect during seven days, and appeared to be much affected by his discourses on the last things, and on the fear of God. Nevertheless, he was not yet converted.
St. Bernard, who had learned never to despair of the most obstinate sinners, redoubled his tears, prayers, and pious endeavours ... but could not prevail upon him to restore the two bishops whom he had unjustly deprived of their sees. At length he had recourse to more powerful arms.
He went to say mass, the duke and other schismatics staying without the door, as being excommunicated persons. After the consecration, and the giving of the peace before the communion, the holy abbot put the host upon the paten, and carrying it out, with his eyes sparkling with zeal, charity, and devotion, and his countenance all on fire, spoke to the duke no longer as a suppliant, but with a voice of authority, as follows:
“Hitherto we have entreated you and prayed you, and you have always slighted us. Several servants of God have joined their entreaties with ours, and you have never regarded them. Now, therefore, the Son of the Virgin, the Lord and head of that church which you persecute, comes in person to see if you will repent. He is your judge, at whose name every knee bends, both in heaven, earth, and hell. He is the just revenger of your crimes, into whose hands this your obstinate soul will one day fall. Will you despise him? Will you be able to slight him as you have done his servants? Will you?”
Here the duke, not being able to hear any more, fell down in a swoon. St. Bernard lifted him up, and bade him salute the bishop of Poitiers, who was present. The astonished prince was not able to speak, but went to the bishop, and led him by the hand to his seat in the church; expressing by that action that he renounced the schism, and restored the bishop to his see. After this, the saint returned to the altar and finished the sacrifice.
It's from Alban Butler's Lives of the Saints, and God only knows how much truth there may be to it. Still, it's a cool story.