Rivaz of the Claw

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.

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.

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.