| Odo I | |
| Succession: | Duke of Brittany, with Alan III |
| Reign: | 1008–1035 |
| Predecessor: | Geoffrey I |
| Regent: | Hawise of Normandy |
| Successor: | Alan III by himself |
| Succession1: | Count of Penthièvre |
| Reign1: | 1035–1079 |
| Successor1: | Geoffrey I |
| Succession2: | Regent of Brittany |
| Reign2: | 1040–1057 |
| Spouse: | Orguen-Agnes of Cornouaille |
| House: | House of Rennes |
| Father: | Geoffrey I |
| Mother: | Hawise of Normandy |
| Birth Date: | c. 999 |
| Death Date: | 7 January 1079 (aged 80) |
| Death Place: | Cesson-Sévigné |
| Place Of Burial: | Saint-Brieuc |
Odo of Rennes (Medieval Breton: Eudon Pentevr, Modern Breton: Eozen Penteur, Latin: Eudo, French: Eudes/Éon de Penthièvre) (c. 999–1079), Count of Penthièvre, was the youngest of the three sons of Duke Geoffrey I of Brittany and Hawise of Normandy, daughter of Richard I of Normandy. Eudon married Agnes of Cornouaille (Orguen Kernev), the daughter of Alan Canhiart, Count of Cornouaille and sister of Hoel II, Duke of Brittany who was married in 1066 to Eudon's niece Hawise, Duchess of Brittany.
When Eudon's father Duke Geoffrey I died on 20 November 1008, both Eudon and his older brother Alan were minors.
Duke Geoffrey had initiated a dynastic double marriage with Richard II, Duke of Normandy by marrying Hawise of Normandy, one of Richard's sisters, in 996; this was followed by the marriage of Geoffrey's sister Judith of Brittany to Richard around the year 1000. Alan and Eudon were thus double-first cousins of Duke Richard II's children, including Richard III, Duke of Normandy and Robert I, Duke of Normandy (William the Conqueror's father), nephews of Emma of Normandy, and first cousins of Emma's children Edward the Confessor, Harthacnut, Goda of England, Gunhilda of Denmark and Alfred Aetheling.
At Geoffrey's death, Alan became de jure Duke, as Alan III, Duke of Brittany, Hawise took on the role of Regent, and Richard asserted the role of Guardian of Brittany, an arrangement reciprocated on the death of Robert I, Duke of Normandy in 1035.
Also in 1035, after a dispute between Eudon and Duke Alan III, their uncle Judicaël Bishop of Vannes arbitrated, and Alan III gave Eudon the bishoprics of Saint-Brieuc, Saint-Malo, Tréguier and Dol-de-Bretagne, as well as the counties and baronies of Penthièvre, Goëlo, Avaugour and Lamballe. Eudon placed his capital at Lamballe, where he began issuing coins in his own name.
Following the death of his brother Duke Alan III in 1040, Eudon ruled as regent of Brittany in the name of his nephew Conan II, holding Conan in custody. Conan was freed by his supporters in 1047. Eudon's regency should have ended when Conan reached his majority (circa 1054), but Eudon refused to relinquish control of the Duchy.
In 1054 Geoffrey, Viscount of Mayenne in Maine, affronted by William of Normandy's new fortifications at Ambrières, "appealed to Geoffrey, Count of Anjou" who "called to his assistance Guy-William of Aquitaine and Count Eudo of Brittany"[1]
In a charter dated to 1056/1060, Eudon ("Eudo") granted land "in pago Belvacensi" (Beauvais, Picardy) to the Abbey of Angers Saint-Aubin (q.v. Albinus of Angers). Witnesses included his wife Orguen and their sons Gausfridus, Alanus Rufus, Willelmus, Rotbertus and Ricardus (Geoffrey, Alan Rufus, William, Robert and Richard).
By 1056 Conan gained the upper hand in Brittany, and in 1057 he captured his uncle Eudon and chained him in a prison cell. Eudon's eldest son Geoffrey Boterel continued to fight against Conan.
In 1062, peace was concluded between Conan and Geoffrey. Eudon, who was now free, continued the fight alone.
In histories favourable to the house of Penthièvre, Eudon is shown as effectively ruling Brittany between 1040 and 1062. In other histories his rule is shown as ending with his capture in 1057.
Conan was a legitimate contender for the title of Duke of Normandy, so he became a serious rival to Duke William.
In 1064, Eudon's liegeman Rivallon I of Dol invited Duke William to join him against Conan, thus initiating the Breton-Norman War of 1064–1065 in which Normandy, Anjou, Dol de Bretagne and the captive Harold Godwinson combined against Conan II, as depicted in three panels of the Bayeux Tapestry. Historians differ on whether William or Conan should be considered the victor in this conflict.
Eudon provided, trained and equipped 5,000 Breton soldiers for William the Conqueror's army. Of these, 4,000 were professionals, comprising light cavalry, heavy cavalry, archers, crossbowmen and axemen; he also trained 1,000 levied (conscripted) spearmen. Eudon put these troops on 100 ships under the command of his sons Count Alan Rufus and Count Brian, and they sailed from Brittany to join the Norman forces gathering at Barfleur then on to William's staging point at Saint-Valery-sur-Somme, in readiness to cross the Channel.
Despite his advanced years, Eudon was involved in the failed 1075/1076 rebellion against Hoel II, Duke of Brittany, by Geoffrey Grenonat of Rennes and Ralph de Gael (fresh from his 1075 rebellion against King William). Hoel II was supported by William the Conqueror, briefly placing Eudon and William on opposing sides once again, a situation that was soon resolved when, to William's disappointment, Hoel came to terms with the rebels.
On 7 January 1079, at about 80 years of age, Eudon died in Cesson-Sévigné, an eastern suburb of Rennes. He was buried in Saint-Brieuc Cathedral.
In his memory, "Comes Alanus Rufus" (Count Alan Rufus), his second son, donated property to Swavesey Abbey in Cambridgeshire, for the soul of "patris sui Eudonis comitis" (his father Count Eudon), by an undated charter witnessed by "...Ribaldus et Bardulfus fratres comitis..." (Ribald and Bardulf, brothers of Count Alan).
Orderic Vitalis stated:[2]
"God also gave him [Count Eudon] seven sons, who became remarkable for the singular and changeable events of their lives. The studious might compose a long and pleasing history, from true accounts of their various fortunes."
Eudon's children with Agnes include:
Sons of Eudon who were probably illegitimate include:
At least two of Eudon's sons (Alan Rufus and Brian) were early participants in the Norman conquest of England.[6]
Eudon's descendants formed the junior branch of the Breton ducal family, which gained control of the duchy in 1156 under his great-grandson Conan IV of Brittany.