The Cauldron comes from Lloyd Alexander's book The Black Cauldron, or perhaps from an older mythical source he may have based it on, or on numerous other sources that borrowed from him.
Alexander did indeed get the Balck Cauldron (the correct name was the Black Crocken) from Welsh mythology, specifically the second branch of the Mabinogian, the story called 'Branwen, Daughter of Llyr'. The first four 'branches' of the Mabinogian are by far the most important modern source of Welsh mythology, and other references come from this 'book'. The Mabinogian itself comes to use today from two medieval manuscripts, but the the stories refer to life around the first century, before the Roman invasion. Remember that Wales became dominated by Celtic invasions, a major one probably refernced being from the Belgae in 279 BC., so we have the 279 BC to the Roman period as the endpoints. The actual date of the original story content is unknown.
The Mabinogian is highly recommended to anyone interested in mythology/folklore, and Celtic themes have very large influence on modern fantasy. Like the Book of Invasions for Irish mythology, the stories refer to euhemerized gods, probably because referring to them as gods when written in a Christian era wouldn't be possible.
In 'Branwen', the giant-king Bran the Blessed was the king of Wales, and his sister Branwen married the King of Ireland Mathonwy. Her relative Efnisien, a horrible guy, is insulted by not being consulted, and starts trouble by killing Mathonwy's horses. Bran offers to restore the losses. Mathonwy and the Irish, however, wouldn't forgive them, and abused Branwen. Branwen finally gets news to Bran, who invades Ireland.
An agreement is made so that Mathonwy's and Branwen's son will rule Ireland. Unfortunately, the bad guy Efnisien kills the boy, and this starts a war. The Irish were going to trick the Welsh, but Efnisein discovers it and kills the hiding men.
At this point, the Irish shouldn't have a chance, but they use the black cauldron to restore their dead warriors. Finally, Efnisien in an act of sacrifice destroys the cauldron but dies doing it. In the Alexander book the Black Cauldron, Arawn (the main bad guy) creates an invulnerable army by putting dead bodies in it, spawning the 'cauldron born'.
The Alexander stories (Chronciles of Prydain) are based on the Welsh tales. Obviously, Ellidyr plays a role in the book The Black Cauldron similar to Efnisien in the Mabinogian.
For any of you who don't know them, Alexander's Prydain series books stories are GOOD!
They are REALLY GOOD! The Black Cauldon and the last book, the High King, are as good as they get, although they are children's stories.
Clearly, the black cauldron in both the Mabinogian and in The Black Cauldron are a lot more powerful than Mokka's cauldron, which otherwise would be very powerful in a game sense.
The tie between the black cauldron in the Mabinogian and the Holy Grail stories (which apparently were invented by the French poet Chretien de Troyes) has been debated, and is based on the Arthurian tradition/Welsh debate.
Best wishes,
Breunor