Pokemon Scarlet and Violet Iron Boulder & Iron Crown: How to unlock and where to catch
Much like with Pokemon Scarlet and Violet: The Teal Mask, new Paradox Pokemon have been added to the game - and catching them follow a similar requirement to the method for unlocking Bloodmoon Beast in the prior DLC. However, there's a twist - while there are 4 new Paradox Pokemon, only 2 are available in Pokemon Scarlet and Pokemon Violet respectively. For Pokemon Violet, the game that we have access to, that would include Iron Boulder and Iron Crown. These are Paradox Pokemon based on Terrakion and Cobalion respectively.
How to unlock Iron Boulder and Iron Crown
Perrin, the same NPC that is involved with the Bloodmoon Beast quest in The Teal Mask, is also involved with the new Paradox Pokemon in The Indigo Disk; she'll be found at the entrance of the Terarium, near the Southeast section of the map. However, she'll only appear if you've already completed her quest in Kitakami by catching over 150 of the Pokemon from the Kitakami Pokedex and helping her with the Bloodmoon Beast. Once you've interacted with her, she'll tease a "hot tip" she has, and will task you with catching 200 Pokemon from the Blueberry Pokedex before she'll let you in on it. Once you've caught the required number of Pokemon she'll hand over two pictures of the Paradox Pokemon, as seen in Area Zero. These Pokemon will not spawn in Area Zero until you've been shown the pictures in question.
Where to catch Iron Boulder and Iron Crown
Once you've received the two photos, these will double as hints for where to find the two Paradox Pokemon. Iron Boulder can be found underneath a collapsed pathway, near the bottom of the upper portion of Area Zero, right before the chasm. It's not too difficult to find, just scan the perimeter and you'll find it in no time.
As for Iron Crown, you'll want to look for the alcove overlooking Research Station 1. This one might be trickier to find, but as a hint it will be directly across from one of the waterfalls dotting the area, as seen below.
Both Iron Boulder and Iron Crown are found at Level 75, and must be interacted with to battle. Once you've caught both of the Paradox Pokemon, return to Perrin and she'll both congratulate you on your accomplishment, and award you a Sports Ball. Maybe not as exciting as the free Hisuian Growlithe you received in The Teal Mask, but hey; you got 2 Paradox Pokemon this time, instead of just the one.