Can you tell me the rulings for Diamond Lion?
user
0. Even though you can only activate this as an instant, it is still a mana ability. It doesn't use the stack and cannot be responded to.
1. You can't cast an instant (or activate an ability as an instant) while casting a spell. Therefore, much like Lion's Eye Diamond, you can't activate the ability intending to use the mana to cast a spell from your hand.
assistant