Twist.
there is a quest to destroy or play 4 old gods.
In the current twist session, there are no “Old God” cards available in order to accomplish the quest.
i even made the only “all” card available (using 800 dust) and the quest did not count that card for completion of the quest.
is there no quality control, or supervision there at Blizzard???
Cthun?? Nzot?? The only yoggy??
Nonsense. Of course there are.
They don’t have “old god” in the actual name or card text, though, so if you search your collection for “old god”, then yeah, you won’t find any results…
Old Gods in Hearthstone are C’Thun, N’Zoth, Yogg and Y’Shaarj
That tribe only exists in Mercenary
Open a Whispers of the Old Gods pack and you will receive C’Thun for free. Then you will have an Old God card to play.
There’s also also G’huun, but not legal in Twist right now.
https://hearthstone.fandom.com/wiki/G%27huun_the_Blood_God
But are you sure he is considered as an olg god in HS ? Because if in theory he is, they may have not implemented that
flavor text would imply that the other old gods wouldn’t let him play this game either lol.
You’ve answered your question yourself: the ‘tribe’ is not implemented, apart from Mercs, and that loser isn’t there. 
By the way, there’s more than one version of the four, isn’t there? Only the WotOG ones eligible for the quest, though.