Sylas had no ability to draw Demonic Runes normally. This was because Earth didn't produce Demonic Aether naturally.
All Runes required Aether to function. So logically speaking, if there was no Demonic Aether, then there was no way Demonic Runes would work.
However, who said there was none? Wasn't the Blackened Claw in Sylas' right hand precisely a mass of Demonic Runes and Demonic Aether?
Sylas flipped over his other palm, splitting his mind to activate Extricate. He pulled the Blood Essence of a random beast from his Madness Key, pouring it into the Blackened Claw and supplementing the Demonic Aether it was using up.
CRACK.
In just three seconds, the barrier had already suffered over four million Damage, and it was only accelerating.
Sylas was drawing the very same Poison Demonic Runes that he had seen in the Woodland Territory, and every time he drew a new one, he became more proficient.