Before thrusting the Godslayer into the brain, he was prepared for all sorts of consequences. It was no ordinary sword but the weapon that killed the goddess. He was now using it to poke into Yggdrasil's brain. No one was going to believe this was surgery. Hao Ren was only able to take up the courage to do because the patient was the First Born, and Godslayer was no longer a deadly weapon.
But he never thought Yggdrasil would stop breathing the moment he pierced the sword into its brain.