Nightorah, having lost his left and right heads was furious at Godzilla, Rodan and Mothra due to them being the reason he lost them.

They really thought they got rid of me, Nightorah? Hah! Ridiculous! Cutting off two of my heads? Burying me alive? That won’t be enough to stop me, you see! I got out! And now it’s time for revenge!

you find Nightorah as he spoke about this revenge of his.