Earlier today, Blizzard took the Lionheart (AU) Classic Season of Mastery realm offline after it was discovered that players were able to transfer Eternal Classic characters over to the realm — allowing fully Naxxramas geared characters to wreck havoc on the Season of Mastery's economy and PvP.
Roughly 20 minutes ago, the realm has been brought back online, with Blizzard announcing that transferred characters were being returned to their origin and their disruptive actions reversed.
UPDATED. Read more on wowhead.com