Here’s how The CW describes the episode:
REIGN CHALLENGES SUPERGIRL – Supergirl (Melissa Benoist) investigates a mysterious symbol popping up all over National City, tracing its origins back to an ancient prophecy and the mark of the World Killer, Reign (Odetee Annable). Meanwhile, the tension between Lena (Katie McGrath) and Morgan Edge (guest star Adrian Pasdar) continues to build, causing James to step in and offer up some protection, and possibly igniting a spark in the process. Glen Winter directed the episode written by Paula Yoo & Caitlin Parrish (#309). Original airdate 12/4/17.