Achievement text says:
Slay 3 unique rank S elite marks in the Coerthas western highlands or the Sea of Clouds.

I suppose it is missing Azys Lla, since every map only has 1 S rank. And normally you would say "Coerthas Western Highlands" too, I think.

On the side note, achievement "Mark of the Dragon: S" has a comma before "or the Churning Mists", which is not normally there.