Johanna Magdalene of Saxe-Weissenfels
Johanna Magdalene of Saxe- Weissenfels( 17 March 1708 – 25 January 1760), was a Duchess consort of Courland.
She married the Duke of Courland, Ferdinand Kettler, on 20 September 1730.
She was the daughter of Johann Georg, Duke of Saxe- Weissenfels and Fredericka Elisabeth of Saxe- Eisenach.
The marriage was childless.
John William III, Duke of Saxe-Eisenach
John William III, Duke of Saxe- Eisenach( 17 October 1666 – 14 January 1729), was a duke of Saxe- Eisenach, and came from the Ernestine line of the house of Wettin.
Albert IV, Duke of Saxe-Eisenach
Albrecht, Duke of Saxe-Eisenach, (Altenburg, 27 July 1599 – Eisenach, 20 December 1644) was a ruler of the duchy of Saxe-Eisenach.
He was the seventh (but fourth surviving) son of Johann, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt.
His regnal name Albert IV derives from the numbering of the duchy of Saxony as a whole, not specifically to the succession in Saxe-Eisenach.
Albrecht received his first instruction from the Field Marshal Frederick of Kospoth.
Later he studied at the University of Jena with his brothers.
In the years 1619-1621 he completed his "Cavalierstour" (Study Tour) with his younger brother Johann Frederick.
The two princes travelled to France and Switzerland.
After his return in 1621, Albrecht occupied himself with administrative duties until 1626.
He also represented his absent brothers as regent.
In Weimar on 24 June 1633 Albrecht married Dorothea of Saxe-Altenburg, daughter of Frederick William I, Duke of Saxe-Weimar.
The marriage was childless.
In accordance with a divisionary treaty concluded with his brothers, Albrecht received Eisenach in 1640.
He died four years later, after which his state was merged with Saxe-Weimar under Wilhelm of Saxe-Weimar.
August, Duke of Saxe-Merseburg-Zörbig
August, Duke of Saxe-Merseburg-Zörbig (Merseburg, 15 February 1655 – Zörbig, 27 March 1715), was a German prince and member of the House of Wettin.
He was the third but second surviving son of Christian I, Duke of Saxe-Merseburg and Christiana of Schleswig-Holstein-Sonderburg-Glücksburg.
John George II, Duke of Saxe-Eisenach
Johann Georg II, Duke of Saxe- Eisenach( Friedewald, 24 July
1665 – Eisenach, 10 November 1698), was a duke of Saxe- Eisenach.
He was the second son of Johann Georg I, Duke of Saxe- Eisenach and Johannetta of Sayn- Wittgenstein.
The death of his older brother, Frederick August—who was killed in battle in 1684 — make him the new heir of the duchy of Saxe- Eisenach.
Two years later( 1686), Johann Georg succeeded his father when he died.
In Kirchheim unter Teck on 20 September 1688 Johann Georg married with Sophie Charlotte of Württemberg.
This union was childless.
When his cousin, the young Duke Johann Wilhelm of Saxe- Jena died( 1690) Johann Georg inherited a part of his duchy, because he was compelled to make a divisionary treaty with the Duke Wilhelm Ernst of Saxe- Weimar, his other cousin and brother- in- law of the late duke.
Johann Georg died suddenly of smallpox, and was succeeded by his brother, Johann Wilhelm.
Marie Elisabeth of Brunswick-Wolfenbüttel
Marie Elisabeth of Brunswick-Wolfenbüttel (7 January 1638 - 15 February 1687), was a German noblewoman member of the House of Welf and by her two marriages Duchess of Saxe-Eisenach and Saxe-Coburg.
Born in Brunswick, she was the second of the three children of Augustus the Younger, Duke of Brunswick-Lüneburg and his third wife, Duchess Elisabeth Sophie of Mecklenburg.
Marie Elisabeth and her older brother Ferdinand Albert are the only surviving children of their parents' marriage.
In addition, they had seven older half-siblings from Duke Augustus' two previous marriages, of whom only four survive adulthood:
Rudolph Augustus, Sibylle Ursula (by marriage Duchess of Holstein-Glücksburg), Clara Augusta (by marriage Duchess of Württemberg-Neustadt) and Anthony Ulrich, all born from Augustus' second marriage with Dorothea of Anhalt-Zerbst.
Wilhelm Heinrich, Duke of Saxe-Eisenach
Wilhelm Heinrich, Duke of Saxe- Eisenach( 10 November 1691 – 26 July 1741), was a duke of Saxe- Eisenach.
He was born in Oranjewoud, the eldest and only surviving son of Johann Wilhelm, Duke of Saxe- Eisenach and his first wife Amalie of Nassau- Dietz.
Wilhelm Heinrich first married Albertine Juliane of Nassau- Idstein( daughter George August, Count of Nassau- Idstein) in Idstein on 15 February 1713.
This marriage was childless.
He married his second wife Anna Sophie Charlotte of Brandenburg- Schwedt in Berlin on 3 June 1723, just eight months after the death of Albertine Juliane.
The second marriage was also childless.
Wilhelm Heinrich acceded to the duchy of Saxe- Eisenach in 1729 upon the death of his father and was succeeded by his second cousin, duke Ernst August I of Saxe- Weimar.
The personal union between Eisenach and Weimar created by this succession was only nominal until 1809, when the two patrimonies were formally united.
He died in Eisenach.
William August, Duke of Saxe-Eisenach
Wilhelm August, Duke of Saxe-Eisenach (30 November 1668 – 23 February 1671) was a duke of Saxe-Eisenach.
He was born at Eisenach, the youngest and only surviving son of Adolf Wilhelm, Duke of Saxe-Eisenach and Marie Elisabeth of Brunswick-Wolfenbüttel.
All his older brothers died before his father, and Wilhelm August, the fifth and youngest child, was born nine days after the death of Adolf Wilhelm (21 November 1668) and succeeded him in the duchy of Saxe-Eisenach from the moment of his birth, under the guardianship of his uncle Johann Georg.
A sickly boy, he died at Eisenach when only two years old, and Eisenach was inherited by Johann Georg.
Fredericka Elisabeth of Saxe-Eisenach
Fredericka Elisabeth of Saxe- Eisenach( 5 May 1669- 12 November 1730), was a German noblewoman member of the House of Wettin and by marriage Duchess of Saxe- Weissenfels.
Born in Altenkirchen, she was the seventh of eight children born from the marriage of John George I, Duke of Saxe- Eisenach and Johannetta, Countess of Sayn- Wittgenstein- Sayn- Altenkirchen.
From her seven older and younger siblings four survive adulthood: Eleonore Erdmuthe Luise( by her two marriages Margravine of Brandenburg- Ansbach and Electress of Saxony), Frederick August, Hereditary Prince of Saxe- Eisenach, John George II, Duke of Saxe- Eisenach and John William, Duke of Saxe- Eisenach.
Karl August, Grand Duke of Saxe-Weimar-Eisenach
Karl August, sometimes anglicised as Charles Augustus( 3 September 1757 – 14 June 1828), was the sovereign Duke of Saxe- Weimar and of Saxe- Eisenach( in personal union) from 1758, Duke of Saxe- Weimar- Eisenach from its creation( as a political union) in 1809, and grand duke from 1815 until his death.
He is noted for the intellectual brilliance of his court.

Question: Who is William August, Duke Of Saxe-Eisenach's maternal grandfather?
Answer: Augustus the Younger, Duke of Brunswick-Lüneburg