The Chair of the Sovereign or Kadeir Teyrnon comes from the The Book of Taliesin XV.
From The Four Ancient Books of Wales.

The Chair of the Sovereign

The declaration of a clear song,
Of unbounded Awen,
About a warrior of two authors,
Of the race of the steel Ala.
With his staff and his wisdom,
And his swift irruptions,
And his sovereign prince,
And his scriptural number,
And his red purple,
And his assault over the wail,
And his appropriate chair,
Amongst the retinue of the wall.
Did not (he) lead from Cawrnur
Horses pale supporting burdens?
The sovereign elder.
The generous feeder.
The third deep wise one,
To bless Arthur,
Arthur the blessed,
In a compact song.
On the face in battle,
Upon him a restless activity.
Who are the three chief ministers
That guarded the country?
Who are the three skilful (ones)
That kept the token?
That will come with eagerness
To meet their lord?
High (is) the virtue of the course,
High will be the gaiety of the old,
High (is) the horn of travelling,
High the kine in the evening.
High (is) truth when it shines,
Higher when it speaks.
High when came from the cauldron
The three awens of Gogyrwen.
I have been Mynawg, wearing a collar,
With a horn in my hand.
He deserves not the chair
That keeps not my word.
With me is the splendid chair,
The inspiration of fluent (end) urgent song.
What the name of the three Caere,
Between the flood and the ebb?
No one knows who is not pressing
The offspring of their president.
Four Caers there are,
In Prydain, stationary,
Chiefs tumultuous.
As for what may not be, it will not be.
It will not be, because it may not be.
Let him be a conductor of fleets.
Let the billow cover over the shingle,
That the land becomes ocean,
So that it leaves not the cliffs,
Nor hill nor dale,
Nor the least of shelter,
Against the wind when it shall rage.
The chair of the sovereign 
He that keeps it is skilful.
Let them be sought there!
Let the munificent be sought.
Warriors lost,
I think in a wrathful manner.
From the destruction of chiefs,
In a butchering manner,
From the loricated Legion,
Arose the Guledig,
Around the old renowned boundary. 
The sprouting sprigs are broken,
Fragile in like manner.
Fickle and dissolving.
Around the violent borders.
Are the flowing languages.
The briskly-moving stream
Of roving sea-adventurers,
Of the children of Saraphin.
A task deep (and) pure
To liberate Elphin.

Kadeir Teyrnon

Areith awdyl eglur.
Awen tra messur.
Am gwr deu awdur.
O echen aladwr.
Ae ffonsa ae ffur.
Ae reom rechtur.
Ae ri rwyfyadur.
Aerif yscrythur.
Ae goch gochlessur
Ae ergyr dros uur.
Ae kedeir gymessur.
Ym plith goscord uur.
Neus duc o gawr nur.
Meirch gwelw gostrodur.
Teyrnon henur.
Heilyn pascadur.
Treded dofyn doehtur.
Y vendigaw arthur.
Arthur vendigan
ar gerd gyfaenat.
Arwyneb ygkat.
Arnaw bystylat.
Pwy y tri chynweissat.
A werchetwis gwlat.
Pwy y tri chyfarwyd
A getwis arwyd.
A daw wrth awyd.
Erbyn eu harglwyd.
Ban rinwed rotwyd.
Ban vyd hyn hoywed.
Ban corn kerdetrwyd.
Ban biw wrth echwrd.
Ban gwir pan disgleir.
Bannach pan lefeir.
Ban pan doeth o peir.
Ogyrwen awen teir.
Bum mynawc mynweir.
Ygkorn ym nedeir.
Ny dyly kadeir.
Ni gatwo vyggeir.
Kadeir gynif glaer.
Awen huawdyl haer.
Pwy enw y teir kaer.
Rwg lliant a llaer.
Nys gwyr ny vo taer
Eissylut eu maer.
Pedeir kaer yssyd.
Ym prydein powyssed
Rieu merweryd.
Am nyt vo nyt vyd.
Nyt vyd am nyt vo
Llyghessawr a vo.
Llyghessawr a vo.
Tohit gwanec tra gro.
Tir dylan dirbo.
Nac eillt nac ado.
Na bryn na thyno.
Na rynnawd godo.
Rac gwynt pan sorho.
Kadeir teyrnon.
Keluyd rwy katwo.
Keissitor ygno.
Keissitor kedic.
Ketwyr colledic.
Tebygafi dull dic.
O diua pendeuic
O dull diuynnic.
O leon luryc.
Dyrchafawt gwledic.
Am terwyn hen enwic.
Breuhawt bragawt bric.
Breuawl eissoric.
Oric a merin
Am teruyn chwhefrin.
Ieithoed edein.
Aches ffyscyolin
Mordwyet merin.
O plant saraphin.
Dogyn dwfyn diwerin.
Dillygein elphin.