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Before Kireina were to arrive in Genesis, Truhan and Celica led their army towards the breach between worlds, connecting Genesis with the Spirit Plane. This wasn't a reckless act, or well, it was, but not without reasons. Armies of millions of Divine Spirits, raging from Great God Ranks to Pseudo Supreme God Ranks flew constantly towards their "territory" the entire area that Kireina, Flora, and Aura protected within the entirety of the World of Genesis.
And their aim? To steal the Yggdrasil Trees Kireina held in her Divine Realm and probably those of Flora as well, all while they constantly attacked allied Realms that were barely holding back, some already having fallen before the might of these strange, semi-transparent entities that weren't at all like the Spirits they remembered about, such as Brontes or Nereid.
Check "The Protagonist's Sister Is Actually the Strongest"!