Question: Who is the father-in-law of John Ernest, Duke Of Saxe-Eisenach?

Knowledge 1: John Ernest, Duke of Saxe-Eisenach
Johann Ernst of Saxe-Eisenach (Gotha, 9 July 1566 – Eisenach, 23 October 1638), was a duke of Saxe-Eisenach and later of Saxe-Coburg.He was the fourth (but second surviving) and youngest son of Johann Frederick II, Duke of Saxony and Countess Palatine Elisabeth of Simmern-Sponheim.His grandfather, Johann Frederick I, had still held the title of Elector of Saxony, but after the Battle of Mühlberg he lost the title to his cousin Maurice, from the Albertine line.His father tried since then to regain the Electorate again for the Ernestine line.For this purpose he accepted an outlawed knight, Wilhelm von Grumbach, with himself, which led finally to the fact that also over his father the anger of the Emperor.Only one year after his birth was besieged the castle of his father in Gotha by troops of the Elector Augustus of Saxony and finally conquered.His father came into imperial prison from the rest of his life.His mother, Johann Ernst and his older brothers had to flee from Gotha.They found first admission with his uncle, the duke Johann Wilhelm of Saxe-Weimar, who took over also the guardianship for the princes -at the same time, he was granted by the Emperor with the lands of his brother Johann Frederick II-.After a short time in Weimar, Johann Ernst, as well as his mother and his brothers, lived in Eisenach and Eisenberg.Later, his uncle Johann Wilhelm of Saxe-Weimar lost the Imperial favour.In the Diet of Speyer (1570), the Emperor decided to restore to the three sons of Johann Frederick II his hereditary rights.In 1572 his older brother Frederick Heinrich died from typhus fever.The same year, by the Division of Erfurt, the decision of the Diet of Speyer was made: The lands of his father were extracted again from the duchy of Saxe-Weimar, and created from them the new "Duchy of Saxe-Coburg-Eisenach".Johann Ernst and his older surviving brother, Johann Casimir, were made rulers of the new country.During there minority, the lands were under the guardianship of the three Elector Princes: Frederick III of the Palatinate (also his maternal grandfather), Johann George of Brandenburg and Augustus of Saxony; also, they took the regency over Saxe-Coburg-Eisenach.The duchess Elizabeth moved to Austria, where she should live themselves in the future in the proximity of her husband, still imperial prissioner.The two young princes, Johann Casimir and Johann Ernst, moved to Coburg, the future residence of his new principality.With only six years, Johann Ernst was separated from his parents forever and entrusted to the education of strange persons.Since 1578 he visited the University of Leipzig then together with his brother.In 1586, after the wedding of his brother with Anna of Saxony, the daughter of the Elector Augustus, the guardianship finalized, and Johann Casimir began, together with his brother, the independent ruling of Saxe-Coburg-Eisenach.Johann Casimir and Johann Ernst governed together the principality for the next ten years; however, Johann Casimir carried the main responsibility for the government as an older brother.Also, in order to had a separated residence from his brother too, Johann Ernst establishes himself in the small town of Marksuhl on 1587.Since 1590 Johann Ernst withdrew himself from the government of the duchy, with his brother completely agreed it, that this should govern the duchy for five years alone, when this time had elapsed and finally agreed (in 1596) with its brother to a new divisionary treaty.The duchy of Saxe-Eisenach was taken by Johann Ernst as independent principality separated from Saxe-Coburg, who remained with Johann Casimir.Thus, Saxe-Eisenach, for the first time in his history, had his own independent political unit became within the Holy Roman Empire.During his first year of reign, Johann Ernst still live in Marsuhl because Eisenach, the new capital of his country, was inhabited and, only with the establishment of his official residencethe citizens began to moved there.In 1598 Johann Ernst created for his duchy his own "Landesregierung" (Federal State Government) and a" Konsistorium".In 1633 his brother, the duke Johann Casimir of Saxe-Coburg, died childless.For this, Johann Ernst inherited Saxe-Coburg, and until his own death he governed both countries in a personal union, but maintains, however, his residence in Eisenach.In Wiener Neustadt on 23 November 1591 Johann Ernst married firstly with Elisabeth of Mansfeld-Hinterort.She died four days after giving birth to their only son:In Rotenburg on 14 May 1598 Johann Ernst married secondly with Christine of Hesse-Kassel.The marriage was happy, but remained childless.With the death of Johann Ernst ended the older line of Saxe-Coburg-Eisenach.His principality was divided (under the rules of the Ernestine line) between Saxe-Weimar and Saxe-Altenburg. [SEP] Knowledge 2: Christine of Hesse-Kassel (1578–1658)
Christine of Hesse-Kassel (19 October 1578 – 19 August 1658) was a German noblewoman member of the House of Hesse and by marriage Duchess of Saxe-Eisenach and Saxe-Coburg.Born in Kassel, she was the tenth of eleven children born from the marriage of William IV, Landgrave of Hesse-Kassel and his wife Duchess Sabine of Württemberg.She was probably named after both her paternal grandmother and aunt (by marriage Duchess consort of Schleswig-Holstein-Gottorp).
Knowledge 1: Christine of Hesse-Kassel (1578–1658)
Christine of Hesse-Kassel (19 October 1578 – 19 August 1658) was a German noblewoman member of the House of Hesse and by marriage Duchess of Saxe-Eisenach and Saxe-Coburg.Born in Kassel, she was the tenth of eleven children born from the marriage of William IV, Landgrave of Hesse-Kassel and his wife Duchess Sabine of Württemberg.She was probably named after both her paternal grandmother and aunt (by marriage Duchess consort of Schleswig-Holstein-Gottorp). [SEP] Knowledge 2: William, Duke of Saxe-Weimar
Wilhelm, Duke of Saxe-Weimar (Altenburg, 11 April 1598 – Weimar, 17 May 1662), was a duke of Saxe-Weimar.Wilhelm was the fifth (but third surviving) son of Johann, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt.He was brother to Bernard of Saxe-Weimar, a successful Protestant general in the Thirty Years' War, and to Ernest I, Duke of Saxe-Gotha (later Altenburg), a successful and well regarded ruler known as "The Pious".
Knowledge 1: John Ernest, Duke of Saxe-Eisenach
Johann Ernst of Saxe-Eisenach (Gotha, 9 July 1566 – Eisenach, 23 October 1638), was a duke of Saxe-Eisenach and later of Saxe-Coburg.He was the fourth (but second surviving) and youngest son of Johann Frederick II, Duke of Saxony and Countess Palatine Elisabeth of Simmern-Sponheim.His grandfather, Johann Frederick I, had still held the title of Elector of Saxony, but after the Battle of Mühlberg he lost the title to his cousin Maurice, from the Albertine line.His father tried since then to regain the Electorate again for the Ernestine line.For this purpose he accepted an outlawed knight, Wilhelm von Grumbach, with himself, which led finally to the fact that also over his father the anger of the Emperor.Only one year after his birth was besieged the castle of his father in Gotha by troops of the Elector Augustus of Saxony and finally conquered.His father came into imperial prison from the rest of his life.His mother, Johann Ernst and his older brothers had to flee from Gotha.They found first admission with his uncle, the duke Johann Wilhelm of Saxe-Weimar, who took over also the guardianship for the princes -at the same time, he was granted by the Emperor with the lands of his brother Johann Frederick II-.After a short time in Weimar, Johann Ernst, as well as his mother and his brothers, lived in Eisenach and Eisenberg.Later, his uncle Johann Wilhelm of Saxe-Weimar lost the Imperial favour.In the Diet of Speyer (1570), the Emperor decided to restore to the three sons of Johann Frederick II his hereditary rights.In 1572 his older brother Frederick Heinrich died from typhus fever.The same year, by the Division of Erfurt, the decision of the Diet of Speyer was made: The lands of his father were extracted again from the duchy of Saxe-Weimar, and created from them the new "Duchy of Saxe-Coburg-Eisenach".Johann Ernst and his older surviving brother, Johann Casimir, were made rulers of the new country.During there minority, the lands were under the guardianship of the three Elector Princes: Frederick III of the Palatinate (also his maternal grandfather), Johann George of Brandenburg and Augustus of Saxony; also, they took the regency over Saxe-Coburg-Eisenach.The duchess Elizabeth moved to Austria, where she should live themselves in the future in the proximity of her husband, still imperial prissioner.The two young princes, Johann Casimir and Johann Ernst, moved to Coburg, the future residence of his new principality.With only six years, Johann Ernst was separated from his parents forever and entrusted to the education of strange persons.Since 1578 he visited the University of Leipzig then together with his brother.In 1586, after the wedding of his brother with Anna of Saxony, the daughter of the Elector Augustus, the guardianship finalized, and Johann Casimir began, together with his brother, the independent ruling of Saxe-Coburg-Eisenach.Johann Casimir and Johann Ernst governed together the principality for the next ten years; however, Johann Casimir carried the main responsibility for the government as an older brother.Also, in order to had a separated residence from his brother too, Johann Ernst establishes himself in the small town of Marksuhl on 1587.Since 1590 Johann Ernst withdrew himself from the government of the duchy, with his brother completely agreed it, that this should govern the duchy for five years alone, when this time had elapsed and finally agreed (in 1596) with its brother to a new divisionary treaty.The duchy of Saxe-Eisenach was taken by Johann Ernst as independent principality separated from Saxe-Coburg, who remained with Johann Casimir.Thus, Saxe-Eisenach, for the first time in his history, had his own independent political unit became within the Holy Roman Empire.During his first year of reign, Johann Ernst still live in Marsuhl because Eisenach, the new capital of his country, was inhabited and, only with the establishment of his official residencethe citizens began to moved there.In 1598 Johann Ernst created for his duchy his own "Landesregierung" (Federal State Government) and a" Konsistorium".In 1633 his brother, the duke Johann Casimir of Saxe-Coburg, died childless.For this, Johann Ernst inherited Saxe-Coburg, and until his own death he governed both countries in a personal union, but maintains, however, his residence in Eisenach.In Wiener Neustadt on 23 November 1591 Johann Ernst married firstly with Elisabeth of Mansfeld-Hinterort.She died four days after giving birth to their only son:In Rotenburg on 14 May 1598 Johann Ernst married secondly with Christine of Hesse-Kassel.The marriage was happy, but remained childless.With the death of Johann Ernst ended the older line of Saxe-Coburg-Eisenach.His principality was divided (under the rules of the Ernestine line) between Saxe-Weimar and Saxe-Altenburg. [SEP] Knowledge 2: William, Duke of Saxe-Weimar
Wilhelm, Duke of Saxe-Weimar (Altenburg, 11 April 1598 – Weimar, 17 May 1662), was a duke of Saxe-Weimar.Wilhelm was the fifth (but third surviving) son of Johann, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt.He was brother to Bernard of Saxe-Weimar, a successful Protestant general in the Thirty Years' War, and to Ernest I, Duke of Saxe-Gotha (later Altenburg), a successful and well regarded ruler known as "The Pious".

Answer: William IV, Landgrave of Hesse-Kassel