2022-09-09 : Once you begin casting a spell from your graveyard, it immediately moves to the stack. Players can’t take any other actions until you’re done casting the spell.

2022-09-09 : The ability that the spell gains as Rivaz’s last ability resolves continues to apply to the permanent that spell becomes after the spell resolves, even if it stops being a Dragon or stops being a creature.

2022-09-09 : The ability that the spell gains only applies to it once it’s on the battlefield. If the spell is countered before it resolves, that ability will not exile it.

2022-09-09 : The casting permission granted by Rivaz’s third ability doesn’t change when you may cast the spell from your graveyard.

2022-09-09 : The last ability triggers when you cast a Dragon creature spell from your graveyard for any reason. It triggers even if you didn’t use mana generated by Rivaz’s activated ability to pay its costs.

2022-09-09 : You can use mana generated by the activated ability to pay an alternative cost (such as an escape cost) or an additional cost to cast a Dragon spell. It’s not limited to paying just that spell’s mana cost.