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The Celestial was sure that they were not there! There was only that one replicate of them which he realized was a replica when he finally killed it, but he had made sure to check and scan the surrounding areas a lot.
And he was right, they were not there, they had gone to this dimensional space and left a replica with a big part of their soul as bait, while they moved to the outside world and trapped the Celestial there, thinking he had finally found them.
And while he slashed away their replica who hopelessly died, the fusion emerged behind him through the help of Aurora's divinity and magic, teleporting behind him!
But what kind of power could allow them to teleport so silently behind his own back?
Check "The Protagnist's Sister Is Actually the Strongest"!