No Alvarr after all. Was all a prank
This site admits to everything about Alvarr coming back was an aprial fools joke made by them. Cult TV saw it and ran with it when they saw it. THIS is why we all need to make sure to quote sources. Sorry everyone! Looks like no Alvarr. Some 'fans' just do not know how to handle a joke.
SOURCE - http://www.simplesite.com/JosephMawle
SOURCE - http://www.facebook.com/pages/BBC-Merlin-Spoilers/120905734645328