Someone need to leave the game? Never fear, that doesn’t mean you all have to quit.
You can ask other users if they want to jump in and replace the leaving player. If you find someone they must click the leaving user in the game followed by “Offer to replace this player”:
If you cannot find someone willing to replace the leaver, a player can be replaced with a controllable vassal house. All players of the game can click the leaver and launch a vote to replace them by a vassal:
Since House Targaryen cannot be a Vassal, Targaryen can only be replaced by another player. However, if House Targaryen has been defeated, i.e. it no longer has any castles or units, House Targaryen no longer has to remain in the game. The only way that the game will then still activate House Targaryen is if House Targaryen still has power tokens left, to use them. House Targaryen can then gift its remaining power tokens as usual, or more recently, put them back into the pool: