Going back to the Galio update, petricite was the demacian Material that blocked magic.
It’s proven that just it being present can dispel/absorb magic with the Galio story.
So why can sylas do his shit?
His abilities are all based on his chains sure, but how can he steal someone’s magical abilities when magic is nullified around him?
How powerful of a spell caster is he? Why can he use magic while being locked with petricite?