Diamond Lion
Artifact Creature — Cat
, Discard your hand, Sacrifice this creature: Add three mana of any one color. Activate only as an instant.
Howard Lyon
Magic Online Promos (PRM)
• #91371Rulings
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.
(wotc • 2021-06-18)
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.
(wotc • 2021-06-18)
Rulings
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.
(wotc • 2021-06-18)
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.
(wotc • 2021-06-18)