Mordechai ben Hillel
Mordechai ben Hillel HaKohen (c. 1250–1298), also known as The Mordechai, was a 13th century German rabbi and posek. His chief legal commentary on the Talmud, referred to as the Mordechai, is one of the sources of the Shulchan Aruch. He died a martyr's death at Nuremberg.
Little is known of the Mordechai's early life. He belonged to one of the most prominent families of scholars in Germany: his grandfather Hillel, on his mother's side was a grandson of Eliezer ben Joel ha-Levi, who was in turn a grandson of Eliezer ben Nathan. Mordechai was also a relative of Rabbi Asher ben Jehiel. He was married to Selda (née?), with whom he had five children.
His principal teacher was Meir ben Baruch of Rothenburg; he was also taught by Perez ben Elijah of Corbeil, Ephraim ben Nathan, Yaakov ben Moshe Levi Moelin (Jacob ben Moses ha-Levi), Abraham b. Baruch (Meir of Rothenburg's brother), and Dan Ashkenazi. In addition to his knowledge of Talmud and Halakha, the Mordechai was also an expert on Hebrew grammar.
About 1291 the family moved to Goslar. But his right of residence there was disputed by Moses Tako, the local rabbi; although the suit was decided in Mordechai's favor, it was conducted with such bitterness that Mordechai left Goslar and settled at Nuremberg. For the next seven years, he operated a Yeshiva there which attracted students from all over Europe. The Mordechai was murdered, along with his wife and children, in the Rintfleisch Massacres; see Nuremberg: Middle Ages.
Magnum opus 
Mordechai, is a great legal (Halachic) work, which acquired wide authority, and was one of the sources of the Shulchan Aruch of Joseph Caro; it has also been printed as an appendix to the Talmud, since the Soncino edition of 1482. The work is written as a compilation of existing halakhic material, and also provides conclusions and results of long discussions in other works. It thus serves both a source of analysis, as well a source of decided law. Mordechai's knowledge of halakhic literature was phenomenal. He quotes the works, and written or verbal responsa, of about 350 authorities; in fact, Mordechai is now the only source for many Ashkenazi authors. He frequently quotes his teacher, Meir ben Baruch, and much of the Mordechai is written in support of the Tosafists, the last of whom were Mordechai's teachers.
The Ashkenazi authorities, as well as those in Italy, were great admirers of Mordechai, and assiduously studied the Mordechai and recognized its authority as a source of Halakha. Moses Isserles lectured on the Mordechai in his yeshivah, and many of his responsa are devoted to questions regarding difficult passages of the book. Its status was such that, the Mordechai is one of the sources of the Shulchan Aruch of Joseph Caro. In Italy and Poland, where the Mordechai was especially studied, a whole Mordechai literature came into existence.
The Mordechai although linked textually with the Halakhot of Rabbi Isaac Alfasi, is, in fact, an independent work. The connection with Alfasi is an external one: single sentences, sometimes even single words, of the Halakot serve as "catchwords" introducing the relevant material found in the Jerusalem Talmud, the tosafot, as well as other codices and compendiums. The Mordechai is published in two forms: glosses to Alfasi's "Halakhot" in various manuscripts, and also as an appendix to the "Halakhot" - the standard form in today's editions of the Talmud.
The work has a "most peculiar history". As the early critics pointed out, the Mordechai was not issued in its final form by the author. He collected the material, but the work was in fact arranged and published by his pupils, partly during his lifetime and partly after his death. Thus within two generations after Mordechai's death there were two entirely different versions of his work, respectively designated as the "Rhenish" and the "Austrian" versions. The Rhenish is the shorter one of the two. These were not merely two different copies of the Mordechai, but are in fact two materially different compendiums. It is thought that the Austrian Mordechai, as found in the manuscripts, is the original form of the work. The version published in the Talmud is the Rhenish with glosses from the Austrian Mordechai, by Rabbi Samuel ben Aaron of Schlettstallt.
Other works 
- Mordechai was the author of Responsa - however, these have not been preserved but are quoted in other works and responsa; see: History of Responsa: Twelfth century.
- He authored several selichot.
- Mordechai b. Hillel b. Hillel, jewishencyclopedia.com
- Rabbi Mordechai Ben Hilel Ashkenazi (circa 4010-5058; 1250-1298), chabad.org
- Mordechai b. Hillel, 1911encyclopedia.org