The Raid on the Otherworld
The Book of Taliesin XXX.
From The Four Ancient Books of Wales

I WILL praise the sovereign, supreme king of the land, 
Who hath extended his dominion over the shore of the world.
Complete was the prison of Gweir in Caer Sidi,
Through the spite of Pwyll and Pryderi.
No one before him went into it.
The heavy blue chain held the faithful youth,
And before the spoils of Annwvn woefully he sings,
And till doom shall continue a bard of prayer.
Thrice enough to fill Prydwen, we went into it;
Except seven, none returned from Caer Sidi

Am I not a candidate for fame, if a song is heard? 
In Caer Pedryvan, four its revolutions; 
In the first word from the cauldron when spoken,
From the breath of nine maidens it was gently warmed.
Is it not the cauldron of the chief of Annwvn? What is its intention?
A ridge about its edge and pearls.
It will not boil the food of a coward, that has not been sworn,
A sword bright gleaming to him was raised, 
And in the hand of Lleminawg it was left.
And before the door of the gate of Uffern [hell] the lamp was burning.
And when we went with Arthur; a splendid labour, 
Except seven, none returned from Caer Vedwyd.

Am I not a candidate for fame with the listened song
In Caer Pedryvan, in the isle of the strong door?
The twilight and pitchy darkness were mixed together.
Bright wine their liquor before their retinue.
Thrice enough to fill Prydwen we went on the sea,
Except seven, none returned from Caer Rigor.

I shall not deserve much from the ruler of literature,
Beyond Caer Wydyr they saw not the prowess of Arthur.
Three score Canhwr stood on the wall,
Difficult was a conversation with its sentinel.
Thrice enough to fill Prydwen there went with Arthur,
Except seven, none returned from Caer Golud.

I shall not deserve much from those with long shields.
They know not what day, who the causer,
What hour in the serene day Cwy was born.
Who caused that he should not go to the dales of Devwy.
They know not the brindled ox, thick his head-band.
Seven score knobs in his collar.
And when we went with Arthur of anxious memory,
Except seven, none returned from Caer Vandwy.

I shall not deserve much from those of loose bias,
They know not what day the chief was caused.
What hour in the serene day the owner was born.
What animal they keep, silver its head.
When we went with Arthur of anxious contention,
Except seven, none returned from Caer Ochren.

Monks congregate like dogs in a kennel,
From contact with their superiors they acquire knowledge,
Is one the course of the wind, is one the water of the sea?
Is one the spark of the fire, of unrestrainable tumult?
Monks congregate like wolves,
From contact with their superiors they acquire knowledge.
They know not when the deep night and dawn divide.
Nor what is the course of the wind, or who agitates it,
In what place it dies away, on what land it roars.
The grave of the saint is vanishing from the altar-tomb.
I will pray to the Lord, the great supreme,
That I be not wretched. Christ be my portion.

This is that most famous of poems, the "Prieddu Annwn," wherein only Arthur and his seven warriors raid Annwfn (the Otherworld) for a magic cauldron.  It stands as a sort of mid-way point between The Mabinogi's Cauldron of Regeneration, and the Holy Grail.