Ashton's body wasn't the only thing reacting to the gas. His summons as well as Lich's undead soldiers were too. The smoke was empowering all of them, pushing them beyond their limits. But changes were more intense for Ashton than for the rest of them.
On the other hand, the Lich rejected the smoke even if it tried to latch on to him. How was he able to do it and why he did it was entirely a mystery to Astaroth, but at the moment he did not bother with it. All of his interest was in the changes within Ashton's body because of the corpsification gas.
Astaroth was worried that inhaling too much gas might damage the rest of Ashton's genes. After all, only the undead genes should react positively to Ashton, but the rest of them should be in danger. Especially the vampire genes.
However, to his surprise, even if the zombie genes had explosive growth, the rest of the genes did not have any issues.
'Three levels? What is this absurd exp gain?'
