Artist's description from DA:I wanted to try my hand at drawing crystal ponies but I couldn't think of a single one mentioned in the show. Then I remembered Radiant Hope from the IDW comics and how freaking awesome she was (I've heard that some fans didn't like her, but I still think she's cool). Now since I drew Hope I needed to draw Sombra as well since it's not right to just draw one and not the other. They were very sweet together in Siege of the Crystal Empire, which also is my all time favorite comic arc.
Though to be honest, before I read that arc, I seriously thought they murdered Sombra at the end of the Crystal Empire episodes in the show. I was glad they didn't since he had such a cool character design, but was so poorly characterized on TV. The other reason why I enjoyed Siege of the Crystal Empire so much was that they finally fleshed out his character and actually had him say more than one or two lines.
Radiant Hope:
I'm not entirely sure why she's got a crown and necklace here, but there was something in the comics about how she could have been a princess. I like drawing royal horses.
* Crystal ponies all have crystals growing from various parts of their body. It's a side effect that happened after living in a place with so many crystals and magic (I was too lazy to attempt to cell shade crystal patterns, so I came up with this instead).
** All crystal ponies are earth ponies, so as a crystal unicorn Hope is only part crystal pony. Mixed blood is shunned by other pure crystal ponies, which is why Hope ended up growing up with Sombra in an orphanage (it's sad, I know, pony racism).
** as a result of her crystal pony blood, her tail is shorter than usual, but she does have crystal fetlocks.
** because of her focus on healing magic, her horn is unusually thin for precision
* Hope has some feathering around her shoulders and tail since she was close to becoming an alicorn princess.
King Sombra:
His reformed design was kind of bland, not to mention he ended up looking like mirror!Sombra, so I just kept his old design and made him less evil looking.
* Like Hope, Sombra's also part crystal pony and part unicorn (and obviously part umbrum), so he's also got crystals growing from his fur
** because of his umbrum magic, his horn's weirdly shaped and crystalline
** Sombra most commonly practices offensive magic, so his horn is thicker and lacks precision as he favors blunt force spells
* Canonically, both him and Hope are off looking for the scattered pieces of Princess Amore, but I imagine they would hang around the Crystal Empire sometimes and try to make up for everything they did there since it is their home after all.
On an unrelated side note, has anyone ever read Wuthering Heights by Emily Bronte? When I read it senior year, I kept associating Heathcliff and Catherine (the elder) with Sombra and Radiant Hope. Their characters are quite similar and they follow similar character arcs (except, you know, one of them is definitely more child-friendly).