Creosote is a Seriph of Klatch from the city of Al Khali who figures in the novel Sourcery. He has a spiky turban and isn't very good at poetry. He is the son of Creosote and one of the richest men alive, attempting to spend his wealth on creating a Pleasure-dome, as per poetic instructions and according to the laws of the Narrative Universe, which are clearly based on the Roundworld poets Samuel Taylor Coleridge's Kubla Khan and Omar Khayyam's The Rubaiyat..
Unfortunately for him, he suffers from a sort of poetic dyslexia where he can see the shapes of poems which elsewhere in the known Multiverse have seared the soul, fired the imagination, and resonated down the ages, but he just cannot get the vocabulary right - it has been noted that he has the poetic sensitivity of a hyena.
His position appears to be a figure head position with no actual executive power as his domain is effectively ruled by the Grand Vizier, Abrim, to whom the Archchancellor's Hat steers Rincewind and Conina.
In the ensuing mayhem, as the Hat sets about using Abrim as a worthy tool to fight the Sourcerer, Creosote's assets diminish considerably as the people take advantage of the chaos to loot the palace.
On a hastily acquired magic carpet, the three, plus Nijel the Destroyer, flee Klatch and eventually end up in Ankh-Morpork for the final showdown.
Even though we are assured that the Sourcerer, as his last act before entering Maligree's Wonderful Garden and closing the door behind himself, restores everything on the Disc to just the way it was before all the trouble, we last see Creosote in Ankh, where he discovers his over-literal and pedestrian poetry is actually appreciated, using his best lines to charm barmaids into giving him free drinks.
The Chronicles are silent as to whether he is ever restored as Seriph, If he is Seriph at the time of Jingo, it is not mentioned, as the focus is on the Klatchian potentates Prince Cadram and Prince Khufurah.