Unlike the dragons, there are no specific riders. You create one of your own choosing, so long as they are human and within the limitations set, and if you have any questions, feel free to ask.
As for your dragon, I do have a few things I'd like to go over with you.
First, I want to note that Darkness is an Element, which means it's technically off limits.
There are TEN Elements on Partheus in two circles: Fire, Water, Air, and Earth lead by Spirit - Light, Darkness, Life, and Death lead by Chaos. These are LIVING CREATURES, though not always physically manifested. They are, like the dragons, children of the goddess, Kallydrana. To possess such a power is rare, and Element powers will only be handed out according to how to RP is played.
So, Dusk can't have an affinity for Darkness as of yet. He might later, but not right now.
As a Hatchling and Young Dragon, Dusk will have practically no control over magma/lava, the power being directly linked to his emotions. By nature, Obsidian Dragons are quick to anger and volatile when it comes to their emotions, so he will have moments where he gets angry over nothing, and can quite possibly create a small hole where lava suddenly shoots out. Not to say you have to change his personality, just be sure to note that it's the nature of the Obsidian to be angry at nothing.
ALSO, I don't know if you noticed his breath weapon, but Obsidians are built to be, in essence, war machines. His scales are going to be sharp and everywhere (though if you have a good reason for his belly being soft, please share - I want you to personalize the dragon, but I also want you to give reasons why you are giving that dragon a specific difference), and from the moment he hatches, he'll be breaking pieces off and swallowing them to store them for future use. Their scales are a lot like a rodent's teeth - they never stop growing...so, Obsidian's break pieces off and store them for an awesome and dangerous breath weapon. It's instinct for them.
Also, could you explain why he's weak to light magic (there's not really holy magic on Partheus, as there really isn't anything holy that would grant that special magic)?? What happens when light hits him? Is it the light of the suns (there are two) or the moons? Is it the Element, itself? (if so, we may have a problem, as the maternal guardian is Xaria, who has a natural affinity for Light - would be interesting to play on, indeed)
Give me details, darling! Go all out! SHOW ME THE LOVE! <3<3<3
PLEASE NOTE:
I'm not trying to intimidate you, I promise. I sometimes end up having that effect on people, which is why many of my RPs go south...but I want you to know that I DO WANT YOU TO BE CREATIVE, but I also have a specific structure to each dragon breed that you should at least follow a little. I like the thought of the magma being purple, and that might end up leading to something in the story. XD But please try to uphold the nature of these dragons, or give me reasons why you're not. <3