@RRobin2016 Well in terms of strength Charybdis is also considered S Rank/Disaster-Class, but it's classified as Special A Rank/Calamity-Class only because it's an instinct-driven beast with no intelligence. That said, Charybdis strength is only on the level of an established Demon Lord Seed like Carrion, Frey and Clayman. In LN vol. 6, Benimaru reached that level of strength and managed to slay another incarnation of Charybdis, so there's that.
However Ruminas/Luminous is a True Demon Lord, and while both the Demon Lord Seeds and True Demon Lords are considered S Rank/Disaster-Class, True Demon Lords have 10x the amount of magicule capacity compared to Demon Lord Seeds and Charybdis, and while that doesn't 1:1 translate into actual combat strength, it still shows that she's many times stronger than Charybdis.