The king named Uther Pendragon first appears in any sort of story in surviving texts in the Historia Regum Britanniae by the 12th century write Geoffrey of Monmouth.

According to Geoffrey's account, King Uther although ill, had himself carried in a horse litter to battle the Saxons. The battle was won, but Uther's illness became worse and he and could only stand to drink from a well near Verulam (St. Albans). Some Saxon spies discovered this and poisoned the well. Uther died of the poisoned water, and a hundred of his followers who drank the water after him. See http://www.lib.rochester.edu/CAMELOT/geofhkb.htm . This account also appears in the derived versions of Wace and Layamon and in other derived versions.

In the Story of Merlin attributed to Robert de Boron, it was Merlin who advised Uther to go with his army in a horse litter. But Merlin tells Uther that he will die following the battle. So it happens. After winning the battle, Uther's illness increases and he dies. See http://quod.lib.umich.edu/cgi/t/text/text-idx?c=cme;idno=Merlin , chapter V, and http://members.terracom.net/~dorothea/baladro/index.html , chapter 17. This version, without the poisoning, is the source of the account in Sir Thomas Malory's Le Morte d'Arthur.