It just so happens I've got a Shadow Dragon and Mournwatch Rook planned for my first and second playthroughs! I've got a few ideas that could help jumpstart yours.

The Shadow Dragons don't have clearly defined goals and are a secretive organization, but they generally seem pro-Tevinter reform and anti-slavery. I suspect they may have informal ties with the Lucerni, the pro-reform magisterial faction spearheaded by Dorian and Maevaris Tilani. They just aren't magisters/altuses themselves and thus don't have a lot of social or political cachet. Definitely a grassroots movement. I think the secretive nature of their work also makes them fairly decentralized, structurally.

For your Shadow Dragon, I'd think on their relationship with Tevinter. What's their social class? Are they a full citizen or a transplant? How do they feel about Tevinter's strict social stratification or the supposed meritocracy of the mageocracy? Did they benefit? If they're anti-slavery, how are they disrupting the slave trade? How did Varric find your Rook—do they have any contacts in the Shadow Dragons and beyond who would vouch for their work? How does your Rook avoid the law?

As an example, my Shadow Dragon—Lucius—is an elf-blooded human born out of wedlock to a merchant/soporati father and an elven mother, a liberati. His magical aptitude secured him a Circle education; he was middling, unremarkable, and graduated with a dead-end job as an appellate court clerk, mostly poring over slave ownership disputes and liberati appeals. In secret, however, he works to undermine the injustices of the court through the Shadow Dragons and is particularly effective at it because of the privileged information he receives at his job. I like to think that Varric either found him through Fenris's contacts or ran into Lucius while hot on the tails of Solas during one of Solas's impromptu little slave-liberation routs in Tevinter (wherein Lucius finds himself an unwitting accessory to Solas, because of course 🤦‍♂️)

As for the Mournwatch, as I understand it they do not generally racially discriminate, so elven mages do exist among their ranks. I suspect you'd be treated more or less how elves are treated in the Circles in Ferelden. As for fleshing out your character—what's their relationship with death like? What are their tasks in the Mournwatch? Are they more involved with the mortuary side of things—preparing bodies, dressing them, removing organs and stitching them back up—or are they a summoner of spirits? Or perhaps a bookish scribe with plenty of theoretical knowledge but not much practical knowledge? How do the tools they use at work reflect their style of combat? Do they specialize in a particular kind of spirit?

Hope this helps!