The bulk update request #8132 has been approved by @evazion.
create alias legendarium -> tolkien's_legendarium
create alias tolkiens_legendarium -> tolkien's_legendarium
"Legendarium" is just an English word, and used on its own like this it suggests the name of a copyright called by the name "Legendarium" (capital L), but this tag is about a specific legendarium (lowercase l), namely that of Tolkien.
Updated