I'm going to be referring to the Dragon Knights in Ego Draconis only, though they do appear in the whole Divinity series!
The game is set in a setting where dragons are ill-respected and seen as terrible creatures. Universally, they are hated. TThe main character is part of an order, the Dragon Slayers, whose purpose is self-explanatory. The order itself was formed after Lucian the Divine was "killed" by a dragon knight.
Dragons are able to make dragon knights themselves, often taking children and raising them. Dragons seem to prefer humans to gift the power to. However, in Divinity II there was supposedly none left. This was of course until the main character obtained the power against their will. Bit awkward being a dragon slayer dragon knight.
Dragon knights can shift into a dragon from at will thanks to being blessed by the spirit of dragons.
One of the major skills is the ability to mind read. Extremely useful in-game - though it uses experience gained as a currency, this is often a small price. Though humans were able to achieve this power as well through their ceremony (which requires dragon essence), it's a learned trait for them.