Friedrich Ferdinand, Duke of Schleswig-Holstein
Friedrich Ferdinand of Schleswig- Holstein- Sonderburg- Glücksburg( 12 October 1855 – 21 January 1934) was the fourth Duke of Schleswig- Holstein- Sonderburg- Glücksburg and became Duke of Schleswig- Holstein in 1931.
Abel, King of Denmark
Abel Valdemarsen (1218 – 29 June 1252) was Duke of Schleswig from 1232 to 1252 and King of Denmark from 1250 until his death in 1252.
He was the son of Valdemar II by his second wife, Berengaria of Portugal, and brother to Eric IV and Christopher I.
As Duke of Schleswig, Abel came into conflict with his brother, King Eric IV, whose murder in 1250 he was suspected of orchestrating.
Upon taking an oath to clear himself of the allegations, he was elected king.
After a short reign, he was killed during a military expedition in Frisia.
Abel's reign was the shortest of any Danish monarch since the 9th century.
He founded a line of Dukes of Schleswig - the "Abel family" - which ruled the Duchy of Schleswig until 1375.
Friedrich, Duke of Schleswig-Holstein-Sonderburg-Glücksburg
Friedrich of Schleswig- Holstein- Sonderburg- Glücksburg( 23 October 1814 in Schleswig, Duchy of Schleswig – 27 November 1885 in Luisenlund, Schleswig- Holstein, Prussia) was the third Duke of Schleswig- Holstein- Sonderburg- Glücksburg.
Friedrich was the second- eldest son of Friedrich Wilhelm, Duke of Schleswig- Holstein- Sonderburg- Glücksburg and Princess Louise Caroline of Hesse- Kassel and an elder brother of Christian IX of Denmark.
Friedrich inherited the title of Duke of Schleswig- Holstein- Sonderburg- Glücksburg upon his childless brother Karl's death on 14 October 1878.
Henry, Duke of Schleswig
Henry, Duke of Schleswig( – August 1375) was Duke of Schleswig( or Southern Jutland, as the Danes call it) from 1364 until his death.
Abel, Lord of Langeland
Abel Abelsøn( 1252 – 2 April 1279), Lord of Langeland, was the third son of King Abel of Denmark, Duke of Schleswig and younger brother of Valdemar III, Duke of Schleswig and Eric I, Duke of Schleswig.
As a member of the ducal family, he held several fiefs in Southern Denmark.
Eric Longlegs, Lord of Langeland
Erik Eriksøn, also known as Eric Longlegs,( 1272 – 1310), Lord of Langeland, was the second son of Eric I, Duke of Schleswig and younger brother of Valdemar IV, Duke of Schleswig.
Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg
Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg (4 January 1785 – 17 February 1831) inherited the title of Duke of Schleswig-Holstein-Sonderburg-Beck in 1816.
He subsequently changed his title to Duke of Schleswig-Holstein-Sonderburg-Glücksburg in 1825 and founded a line that includes the Royal Houses of Denmark, Greece, Norway, and the Commonwealth realms.
Wilhelm Friedrich, Duke of Schleswig-Holstein
Friedrich, Duke of Schleswig- Holstein- Sonderburg- Glücksburg, from 1934 Duke of Schleswig- Holstein( 23 August 1891 – 10 February 1965), was the Duke of Schleswig- Holstein and Head of the House of Oldenburg from 21 January 1934 until his death on 10 February 1965.
Eric I, Duke of Schleswig
Eric I Abelsøn  (died 27 May 1272) was a Danish nobleman.
He was the ruling Duke of Schleswig from 1260 until his death in 1272.
He was the second son of King Abel of Denmark, Duke of Schleswig and Mechtild of Holstein.
Albert, Duke of Schleswig-Holstein
Albert, Duke of Schleswig- Holstein( Albert John Charles Frederick Alfred George; 26 February 1869 – 27 April 1931), was a grandson of Queen Victoria.
He was the second son of Victoria's daughter, Princess Helena, by her husband Prince Christian of Schleswig- Holstein.
He was the head of the House of Oldenburg and also the House of Schleswig- Holstein- Sonderburg- Augustenburg and the Duke of Schleswig- Holstein between 1921 and 1931.

Question: What is the date of death of Eric I, Duke Of Schleswig's father?
Answer: 29 June 1252