So, just to clarify, there seem to be 2 documentaries "titled" Master of Horror.
1) Circa 2010, or after. I cant figure out what it is exactly, and is available on youtube : MASTER OF HORROR- STEVEN KING- DOCUMENTARY.
I actually wouldnt be surprised if "Master of horror" isn't its actually title, but the one that the person that shared it on youtube gave it
Maybe Marsha would have some kind of ideas about what it is?
2) Released at the end of august 2018, on Channel5, in the UK.
Now being shown in France (Paramount Channel France) and in the USA > Stephen King: Master of Horror - REELZChannel
The first is probably the Biography Channel International episode. From around 2012, I think.