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The power of the Slime Family was mighty, the group quickly slashed through their enemies, the Undead Demon Gods who were supposed to be as strong as Pseudo Supreme Gods were of no problem to them! But why were they so powerful? Aside from the incredible quantity of buffs they were loaded with thanks to Rimuru's Path Jewels that enhanced Slime-type allies amazingly higher compared to others, and even allowed her to summon them to battle quickly, there was something else to it! Kireina herself had witnessed when it happened, and it was something that everybody also went through, becoming a Pseudo Supreme Deity themselves.
Check "The Protagonist's Sister Is Actually the Strongest"!