Both are:
+Female-centric
+Reverse Harem (Multiple MLs)

ML in TMWTCR is King who want to gather power so can make peaceful life in country especially to FL. His personality is kinda like CX.
SML is a villain with two different personality, Which he use his different identities to trick others and run his plans. His personality and love story is kinda like XL.
TML is FL's bodyguard with unspoken feeling who is always around FL.

From same author in same universe.
Dashing Youth is prequel of The blood of youth and all of these dramas are in same universe.

Both dramas are in same universe.
Tales of dark river is spin off of blood of youth and tells a story about assassin organization that had some parts in original story.