Rhongomyniad first appears in the Welsh prose tale Culhwch and Olwen, composed in the 11th or early 12th century and preserved in two medieval manuscripts, the White Book of Rhydderch and the Red Book of Hergest. The tale is a wild comic adventure in which Culhwch, a prince cursed to marry only the daughter of the giant Ysbaddaden, comes to Arthur's court at Celliwig in Cornwall to ask for help. Arthur greets him with extravagant generosity, promising him anything in the world as far as the wind dries, the rain wets, the sun rises, the sea stretches, and the earth extends. Then comes the crucial word eithyr, "but," and Arthur lists the things he will not part with: his ship, his mantle, his sword Caledfwlch, his spear Rhongomyniad, his shield Wynebgwrthucher, his dagger Carnwennan, and his wife Gwenhwyfar. The list is a catalogue of Arthur's identity, and the spear stands at the heart of it, the weapon that a warlord would no more give away than his wife or his sword. The name itself is Welsh, compounded of rhon, meaning spear or lance, and a second element that scholars have debated for generations. Some translate it as "slaying spear" or "striking spear," from a root meaning to strike or slay. Others suggest "cutting spear," from a root meaning to cut or cleave. A more recent and intriguing proposal connects the second element to the Breton word comyn, meaning crooked or bent, which would make Rhongomyniad the "crooked spear," a weapon with a shaft that was not perfectly straight, perhaps deliberately curved for throwing or for use from horseback.

The Welsh Triads, the collections of Welsh lore arranged in groups of three that were compiled and transmitted from the early medieval period onward, elevate Rhongomyniad from a personal possession to a sacred gift. In the triad of the Twenty-Four Knights of Arthur's Court, the text records that Arthur triumphed over every nation in the world through the sacred weapons that God had given him: Rhongomiant his spear, Caledfwlch his sword, and Carnwennan his dagger. This is a Christian framing of what were probably older pagan weapons, the arms of a Celtic warlord rebaptised as gifts from the Christian God. The triad makes clear that Arthur's victories were not won by his own strength alone but by divine favour expressed through his arms, and Rhongomyniad shares in that sanctity even though the Welsh texts never attribute any supernatural power to it. Unlike Caledfwlch, which shines with its own light, or Carnwennan, which Arthur uses to slice the witch Orddu in half, Rhongomyniad is simply a spear, long and broad-bladed and fit for slaughter.

When Geoffrey of Monmouth wrote his Historia Regum Britanniae around 1136, the book that transformed Arthur from a Welsh folk hero into the king of all Europe, he took the Welsh name and shortened it to Ron, presumably an abbreviation of the original Rhongomyniad that his Latin prose could not comfortably accommodate. Geoffrey places the spear in Arthur's right hand at the Battle of Mount Badon, the great victory over the Saxons that the earlier Welsh sources had already made Arthur's most famous achievement. In Geoffrey's account, the Saxons under their leader Cheldric have broken faith with Arthur, renouncing a treaty that would have sent them back to Germany, and have laid siege to the city of Bath. Arthur marches from Scotland, where he had been campaigning against the Picts, and arrives to find the Saxons drawn up on a hill. Before the battle, St Dubricius, the Archbishop of the City of the Legions, preaches a sermon promising instant absolution to any man who dies in the fight, and Arthur arms himself. Geoffrey describes the king putting on a leather jerkin, a golden helmet with a dragon crest, his shield Pridwen painted with the image of the Virgin Mary, his sword Caliburn forged on the isle of Avalon, and in his right hand his lance named Ron, long, broad in the blade, and thirsty for slaughter. The phrase is Geoffrey's own, a Latin poet's flourish, but it captures the essence of the weapon: not a wand of wonders but a tool of war, hungry for Saxon blood.

The Norman poet Wace, who adapted Geoffrey's Latin prose into French verse around 1160 in his Roman de Brut, keeps the name Ron and adds detail. In Wace's version, Arthur carries his lance named Ron at the battle of Bath, sharp at the head, tough and great, and very welcome at need in the press of battle. Wace's Arthur is more chivalric than Geoffrey's, more courtly and less barbaric, but the spear remains what it was: a practical weapon for a practical king. The English poet Layamon, writing his alliterative Brut around 1200, goes further. In a passage describing Arthur's duel with the Roman tribune Frollo outside the walls of Paris, Layamon records that Arthur's spear was made in Caermarthen by a smith called Griffin and had formerly belonged to Uther Pendragon, Arthur's father and predecessor as king. This detail, which appears nowhere in Geoffrey or Wace, may reflect a genuine Welsh tradition that Layamon had access to, or it may be his own invention, but it deepens the spear's significance by making it an heirloom, a weapon passed from father to son like a crown or a kingdom. The name Griffin for the smith is interesting too, for it echoes the Welsh word gryffyn, meaning a strong or powerful man, and suggests that Layamon was working from Welsh material now lost.

Rhongomyniad has no place in the later French and English Arthurian romances. The Vulgate Cycle, the great prose romance of the 13th century that established the canonical Arthurian narrative, gives Arthur a sword from the Lady of the Lake, a scabbard worth ten of the sword, and a Round Table, but it never names his spear. Malory's Le Morte d'Arthur, the 15th-century compilation that became the standard English account, mentions a sword called Clarent that Mordred steals, and Excalibur that Bedivere returns to the lake, but no spear at all. The Welsh weapon, named with pride in the oldest texts, was swallowed by the romance tradition that preferred magical swords and holy grails to a plain spear forged by a smith in Carmarthen. Yet the name survived in the scholarly tradition, and modern Arthurian scholarship has returned to it as evidence of the Welsh roots of the legend, the layer of Celtic warlord poetry that lies beneath the courtly surface of the later romances.