Wily as the Fox achievement

[Joba]

Does this achievement require you to do it with William or can it be done with his heirs?

Thanks :)

[Genghis San]

the achievement says "Starting as Robert "the Fox" so it can be done with his heirs.
i am currrently going for this achievement too and I am hoping it fires as soon as as I am King of Siciliy and Epirus and all vassals are Catholic. The description is not very clear. I am King and rule all counties in Kingdom of Epirus (converting them as I type) but the Pope has grabbed three provinces in the Kingdom of Sicily.
So my question is if I have to defeat the Pope (to rule all of Sicily) or if it is enough to convert all the counties in Sicily and Epirus to Catholicism?

Will let you know if I find out,