Yezidi. Chambers definition -sect in Armenia and the Caucasus who believe in a Supreme God but also pay respect to the devil, believing him to have been reinstated as chief angel ( called by them Yazid), and other minor gods. ( also Yezdi, Yezidee, Zezidee etc)
Oxburgh, I don't have a better answer and quite possibly Bookwormer knows something that I don't know, but...I think Ystradgynlais is a market town in Powys. Could I suggest you wait before sending it off and see if anyone else comes up with another answer.
I must apologise oxburgh, must have had a senior moment, I think kettledrum is right, hope you haven't posted it, I am now trying to find the right answer! All I can find is Ystrad Mynach which seems just to be the HQ of the Rhymney Valley district, so I am now struggling! Sorry again