"Delta" is not an actual Time Lord. I'm brainstorming different ideas on how to represent the Doctor in my set. If I can create a flavorful, splashy card, I'll slot it in as a mythic. See also: Alpha, Beta, Gamma, and Epsilon.

Delta, like Gamma, captures his vengeful side. This was originally going to trigger whenever a creature you control leaves the battlefield, but I think it's much more visceral triggering whenever one of your creature dies.