Today marks the fifth anniversary of World of WarCraft and, subsiquently, the fifteenth anniversary of the entire WarCraft franchise. To celebrate the anniversary of the mastodon of all MMO games, Blizzard is giving players who sign in to their account between now and December 6th a special Onyxia Whelpling pet that will never be available in-game again.
So if you have an active World of WarCraft account make sure to pop in between now and the ending date to get your free Onyxia Whelping. Make sure you do so on all characters as well so you can get your Feat of Strength achivement for just logging on during the anniversary.