Xel'Naga Keystone

{t}: Add up to X mana of any color where X
is the number of counters on Xel'Naga Keystone. Spend this only to cast an artifact, a Xel'Naga or create Xel'Naga.
When Xel'Naga Keystone enters the battlefield put X counters on it where X is
the amount of mana spent to cast this spell.