Wall of Sevens

During each player's upkeep, put an age counter
on Wall of Sevens unless the player pays seven life. If a seventh age counter would be placed on Wall of Sevens, exile Wall of Sevens instead. Return Wall of Sevens to the battlefield at the
end of turn. If Wall of Sevens would be placed in
a graveyard to exile for more that one turn, return Wall of Sevens to the battlefield, unless there are exactly seven creatures on the battlefield at the end of the turn.