Rex, with the aid of a couple of items from the System, was able to create this situation.
He had seen how Meloriana was making a big deal—about the Great Luna's remains, even to Yulthar and was curious why that was the case. Naturally, he did the most logical thing—and asked the System about it.
As it turns out, the Great Luna's remains were an additional source of power for Meloriana.
Upon death, the Great Luna's body undergoes a miracle.
She turned into a powerful artifact capable of turning the normal moonlight energy, and also kingly energy into divinity. It was only connected to Meloriana—since she was the deity that represented love and serenity amongst the Lunirich Gods.
Then again, her strength was also the strength of the entire Lunirich Gods.
If she became stronger then so will the others.
But this doesn't surprise Rex at all.
Before he attacked and destroyed the Bloodstone Crater, he already learned what it was.