The tzaddik Rabbi Yaakov Orenstein zt”l, the Yeshuos Yaakov, lived in the city of Yeroslav, in which the following story occurred (this story was adapted by Rabbi Eliyahu Hoffman):

One Erev Yom Kippur, as the Jews of Yeroslav arrived in the shul and donned their talleisim in preparation for Kol Nidrei, an unfamiliar Jew walked in. His entire being was cloaked in fear and awe, and to the astonishment of the Kahal, proceeded immediately to the Chazzan’s shtender and began to sing Kol Nidrei. Not wanting to create a stir on the holiest day of the year, the shul’s board discreetly decided not to forcibly remove him. After completing Kol Nidrei, the unknown guest proceeded to daven Maariv. After Maariv, he continued standing as he chanted the piyutim, and then, still on his feet, began to recite all of sefer Tehillim.

By the time his emotional and tear-soaked recitation of Tehillim was over, the first rays of sun were already flickering over the horizon. The mysterious and holy guest wasted little time in beginning Shacharit prayers. When the sefer Torah was removed, he lained from it himself, and returned to his spot at the shtender to lead Mussaf. By this point, the Kahal was not resenting the uninvited intrusion, and were busy wondering how anyone could display such stamina and endurance. Over the entire period of time, his prayers had lost none of their intensity, nor their unworldly sweetness. Indeed, some congregants began to whisper that their mystery guest could surely be none other than Eliyahu ha-Navi, or perhaps a heavenly angel!

The Yeshuos Yaakov would later explain:

I myself became caught up in the man-or-angel question. After the arousing Mussaf prayer, he went straight to Mincha and then to Neilah, and, during the last kaddish, blew the shofar himslef and led the Kahal in the most amazing post-Yom Kippur Maariv. By this point, there was a general consensus that an angel had been sent from heaven in order to arouse the Jews of Yeroslav to teshuva, for it seemed humanly impossible for one of flesh and blood to have put on the display that they had just witnessed over the last 24 hours.

After Maariv, my father-in-law approached him and invited him to his house. Not wanting to miss what transpired, I went along. My father-in-law asked his esteemed guest to lead the household in havdalah, which he did, with his characteristic fervour. He drank some wine, and sat down, saying that he felt weak. He asked that they bring him something to strengthen his heart. Could it be that our angel was no angel after all?

It seems, however, that no matter what foods he was presented, he displayed no interest. Eventually, it became clear that it was not food and drink that he desired but a Sukkah Gemara, which was placed before him on the table. He began to learn with great joy and enthusiasm, and I concealed myself underneath a bed in the room to see what would transpire. All night long he learned with great love and eagerness, completing the entire tractate, and leaving immediately afterward to daven Shacharit.

Some time later, we realized that the identity of our holy guest was that of the great Rabbi Levi Yitzchak of Berditchev, the Kedushas Levi zt”l.