I visit Saint Mungo's Holy-stain in Simonburn Church, Northumberland

Saint Mungo (aka Kentigern) meets, debates and baptises Merlin (aka Lailoken) as depicted in Stobo Kirk, Scotland 

Saint Kentigern (aka Mungo) was one of the great missionaries to the Ancient Britons (including Picts), founder of the Cathedral and Patron Saint of the city of Glasgow; and has many churches named after him in Scotland and the North of England. 

One of these is in the remote-isolated Northumbrian church of Simonburn, just north of Hadrian's wall; whose Parish covered the largest area of any in England; and was also/therefore one of the very richest "livings" (i.e. the priest got an enormous "salary") from Medieval right through to Victorian times. 

As you can see from the above inscription; after St Mungo was driven from Glasgow, he went South to the Lake District, then East along Hadrian's Wall - doing missionary work as he went. 

In Simonburn; Mungo baptised in a particular spring of water, upon which the current church was built.

And to this day, the spring wells up at the end of the aisle; creating a damp, stained-patch on the flagstones that has been regarded as Holy ground - as I point-out below: