The death of Cúchulainn was brought about by the scheming of Queen Medb, whose pride had been deeply wounded at her defeat in the War of the Brown Bull, she sought to get her revenge on him. She held meetings with the families of those slain by Cúchulainn, she kept on at them to seek revenge for the deaths of their loved ones against the Champion of Ulster.
Her deadliest weapons were the three daughters of her magician Calatin (who had been killed by Cúchulainn in the previous War) whom she had sent to Alba (Scotland) and Babylon to learn the magical arts. When the three had returned from their training they were very well versed in the magical arts, and could summon up illusions of battle hosts with their skills.
She waited again until the Curse of Macha fell upon the men of Ulster so that Cúchulainn would have to face the forces of magic and her army alone once again.
As the forces of Queen Medb once again marched on Ulster, King Conchobar Mac Nessa called a council of war. His warriors and druids were too incapacitated to fight, however the King did not want Cúchulainn to fight alone for it was known that if the Champion fell, then the land would be luckless forever.
To keep Cúchulainn occupied and unaware of what was transpiring around him, King Conchobar ordered the women, the bards and the poets to divert his attention in every way possible and to keep him idle at Emain Macha. However while this was under way and Cúchulainn was engaged in feasting and enjoyments, the three daughters of Calatin appeared and they created the illusory army out of grass, thistles and withered leaves, and caused the sound of trumpets and the roar and scream of battle to be heard all about Emain Macha.
Cúchulainn had to be convinced it was all an illusion by King Conchobar's druids to stop him from running into battle immediately. King Conchobar realized he would have to protect Cúchulainn from further enticements and trickery, he was moved to the magical valley of Glean-na-Bodhar which means 'Valley of the Deaf', once inside the valley nothing from the outside world can be heard.
The daughters of Calatin worked their magic once more creating the illusion that a vast army surrounded the valley with fires burning and women shrieking and crying. This illusion was so powerful that the noise of it even reached into the valley and although the druids and women attending Cúchulainn tried to drown it out, Cúchulainn heard it and again wanted to rush out into battle and once more Cathbad the druid managed to calm him down sufficiently to convince him that it was all an illusion yet again and that he need not venture into battle.
However the last trick to be played on Cúchulainn succeeded. One daughter of Calatin took on the form of a previous lover of Cúchulainn's and cried and begged with him saying Ulster was being ravaged and what was he doing? sitting and playing and carousing while all about him burned and was destroyed. Cúchulainn was stung into action and neither the women nor the druids could restrain him this time as he ordered his chariot to be harnessed and rode out to find this invading army.
There were many bad omens preceding this last battle. The Gray of Macha his horse refused to be bridled and wept tears of blood; Dechtire his mother brought him wine three times and each time it turned to blood when he tried to drink it. When he crossed the first ford in the river he saw a woman of the Sidhe who was washing clothes and armour, she turned to him and said 'I am washing the armour of Cúchulainn, who rides to his death..'
The champion then came across three old crones roasting a hound on rowan spits. They asked him to partake in their humble meal, but there was a geas on Cúchulainn forbidding him to eat the flesh of the hound (his totem animal) and also against eating meat cooked over an open fire. Cúchulainn at first refused to eat the meat, but the crones persisted saying 'you are too proud to eat an honest meal from a few old women but will feast on rich foods in the halls of chieftains and kings.' Then Cúchulainn took the meat in his left hand - going against the double taboo and as soon as he ate the food he was paralyzed in the left side of his body. The Three Crones were the Morrigan disguised.
It had been predicted that the three spears that Cúchulainn carried into battle would each kill a king. His enemies thus wanted to get these spears out of his possession and into their own so that they might gain the advantage of the prediction. Three Druids from the opposing army were sent to ask for the three spears as it was considered highly unlucky and dishonourable to refuse a request from a Druid.
The first Druid strode up to Cúchulainn and demanded his spear or else he would satirize him, Cúchulainn replied 'never let it be said that I lack generosity, take this spear' and with that he flung it at the Druid and killed him. Lugaid son of Cúroi, took the spear from the Druid's body however and slew Laeg the charioteer of Cúchulainn. This was a terrible loss for Cúchulainn.
Now the second Druid came forward and asked for the spear saying that he would satirize the whole province of Ulster if he was refused the gift. 'I need not give more than one gift a day according to custom, but never let it be said that I allowed Ulster to be satirized for lack of generosity take the spear!' and with that he flung the spear into the Druid's head and killed him. However Erc the King of Leinster snatched the spear, and killed Cúchulainn's horse the Gray of Macha with it.
The third Druid then approached Cúchulainn and said 'Give me your spear or I will satirize your Clan'. Cúchulainn once again rose to the bait and replied 'My family will not be dishonoured because of my selfishness take the spear!' and he flung it into the Druid's heart and killed him. Lugaid son of Cúroi again reclaimed the spear and this time flung it at Cúchulainn himself, wounding him fatally. Thus was the prophecy of the spears fulfilled for Laeg was king of the charioteers, the Gray of Macha was king of the horses, and Cúchulainn was king of the champions.
In his final death pangs Cúchulainn asked that he be allowed to go to the lake to get a drink and then return to the battle. His request was granted and he went to the lake where he bound himself to a standing stone by the lakeside so that he might die standing up like a warrior for he was losing all power in his legs. The hero light was fading from Cúchulainn and his face became as white as snow, finally a crow (the totem of Morrigan goddess of death) came and perched on his shoulder. His enemies still slightly afraid to approach the great champion knew for certain he was now dead and cut off his head as a trophy.
With the death of Cúchulainn the power and prosperity of Emain Macha failed as did the fortunes of the army of the Red Branch of Ulster, as predicted centuries before.