I noticed Why did Riddick save the pup? was tagged riddick, but this seems to violate the general SFF taxonomy. The full series is called The Chronicles of Riddick, which is also (confusingly) the title of the second film.
I propose making riddick (2) and pitch-black (1) synonyms of the-chronicles-of-riddick and editing the tag to indicate that this one tag covers the entire series. Burninating the tags wouldn't stop people from creating them in the future.