Question: Which 991 CE battle beside River Blackwater is the inspiration for an Old English poem of the same name?
Context: Sellic Spell (] , an Old English phrase meaning "wondrous tale" and taken from the poem "Beowulf") is a short prose text available in Modern and Old English redactions, written by J.R.R. Tolkien in a creative attempt to reconstruct the folktale underlying the narrative in the first two thousand lines of the Old English poem "Beowulf".  Among other things, it seeks to clarify and integrate a number of narrative strands in the Anglo-Saxon poem.  The resulting text is a loose variant of the "Skilful Companions" type of folktale, in which each of several characters (two or three in Tolkien's text, depending on the redaction) has a valuable but specific skill.  Unlike in folktales of that type, however, the skills of Tolkien's characters do not supplement each other in the resolution of the narrative problem.  Tolkien's recasting of the material also incorporates the sluggish youth motif and the abandonment of the hero at the waterfall, both elements found in the analogous Old Icelandic "Grettis saga".  The suggestion that a waterfall like that of "Grettis saga" was part of the original setting of the pool of monsters in "Beowulf" was made by W.W. Lawrence in 1912. The Battle of Maldon is the name given to an Old English poem of uncertain date celebrating the real Battle of Maldon of 991, at which the Anglo-Saxons failed to prevent a Viking invasion.  Only 325 lines of the poem are extant; both the beginning and the ending are lost. Daniel is an anonymous Old English poem based loosely on the Biblical Book of Daniel, found in the Junius Manuscript.  The author and the date of "Daniel" are unknown.  Critics have argued that Cædmon is the author of the poem, but this theory has been since disproved.  "Daniel", as it is preserved, is 764 lines long.  There have been numerous arguments that there was originally more to this poem than survives today.  The majority of scholars, however, dismiss these arguments with the evidence that the text finishes at the bottom of a page, and that there is a simple point, which translators assume indicates the end of a complete sentence.  "Daniel" contains a plethora of lines which Old English scholars refer to as “hypermetric” or long.  Daniel is one of the four major Old Testament prophets, along with Isaiah, Jeremiah, and Ezekiel.  The poet even changed the meaning of the story from remaining faithful while you are being persecuted to a story dealing with pride, which is a very common theme in Old English Literature.  The Old English, Daniel is a warning against pride and there are three warnings in the story.  The Israelites were conquered because they lost faith in God, who delivered them from Egypt, and started worshiping idols and this is the first prideful act.  The second and third warnings are about internal pride, shown to Nebuchadnezzar through Daniel's dream interpretations. The Blackwater Valley Path follows the River Blackwater from its source in the Rowhill Nature Reserve in Aldershot to just beyond the point where it joins the River Whitewater near Swallowfield to become the Broadwater.  This long-distance route covers 23 miles/37 km and closely follows the River Blackwater using public rights-of-way as well as permissive footpaths and is waymarked.  It is looked after by the Blackwater Valley Countryside Partnership. The Battle of Maldon took place on 10 or 11 August 991 CE near Maldon beside the River Blackwater in Essex, England, during the reign of Æthelred the Unready.  Earl Byrhtnoth and his thegns led the English against a Viking invasion.  The battle ended in an Anglo-Saxon defeat.  After the battle Archbishop Sigeric of Canterbury and the aldermen of the south-western provinces advised King Æthelred to buy off the Vikings rather than continue the armed struggle.  The result was a payment of 10,000 Roman pounds (3,300 kg) of silver, the first example of Danegeld in England. The Battle of Brunanburh is an Old English poem.  It is preserved in the "Anglo-Saxon Chronicle", a historical record of events in Anglo-Saxon England which was kept from the late ninth to the mid-twelfth century.  The poem records the Battle of Brunanburh, a battle fought in 937 between an English army and a combined army of Scots, Vikings, and Britons.  The battle resulted in an English victory, celebrated by the poem in style and language like that of traditional Old English battle poetry.  The poem is notable because of those traditional elements and has been praised for its authentic tone, but it is also remarkable for its fiercely nationalistic tone, which documents the development of a unified England ruled by the House of Wessex. The River Brain is a tributary of the River Blackwater in Essex, England.  It gives its name to the town of Braintree, although Braintree lies on a low ridge between the Brain and the River Blackwater.  To the north of Braintree it is known as Pods Brook.  The brook rises near the village of Bardfield Saling.  Below Braintree the Brain joins the Blackwater in Whet Mead in Witham. The R666 road is a regional road in Ireland which runs west-east from the R639 in Fermoy town centre to the bridge over the River Blackwater on the outskirts of Lismore.  The road passes through Ballyduff, and beneath the River Blackwater viaduct which carries the M8 motorway.  The R666 begins in Fermoy as the Rathhealy Road.  From here to its merger with the R667 road 3km south of Kilworth, the R666 was once part of the historic road linking Dublin and Cork.  It was mapped as such by Herman Moll in his 1714 "New Map of Ireland". River Tall is a small river in Northern Ireland which joins the River Blackwater just south of Verner’s Bridge and beside the Ardress House.  It is navigable for 4 km by dinghy or canoe.  The Callan River, flowing by the city of Armagh, and the Tall River, running by Ballyhegan, join together, and the united steam enters the Blackwater one mile below Charlemont. The Homecoming of Beorhtnoth Beorhthelm's Son is the title of a work by J. R. R. Tolkien that was originally published in 1953 in volume 6 of the scholarly journal "Essays and Studies by Members of the English Association", and later republished in 1966 in "The Tolkien Reader".  It is a work of historical fiction, inspired by the Old English poem "The Battle of Maldon".  It is written in the form of an alliterative poem, but is also a play, being mainly a dialogue between two characters in the aftermath of the Battle of Maldon.  The work was accompanied by two essays, also by Tolkien, one before and one after the main work.
Answer: Battle of Maldon