Just a guess - but it seems that Metamorphosis is a way for a warlock to _become_ the VoidWalker so that they could tank an instance. I'm guessing at least 5-persons/heroics, and perhaps even serve as an offtank in some raid encounters. It seems strange to give a class a new role (though warlocks traditionally tank in Leo and other fights). Whether or not this talent goes live its presence in the Alpha shows that apparently nothing is off the table.
EDIT: Whoops! 45sec duration. I leave the post as a testament to my stupidity. Eek!