@maxal, so far we know of people with a fraction of Shard's power that could push planets around, tilt its crust, flatten and raise mountains, genetically alter masses of people, create a whole new species, develop microorganisms for a specific task, modificate biosphere to be able to survive in ash-filled atmosphere etc.
...
and it's causing autism that makes you draw the line of "that's impossible"? That Nightwatcher - that seems to be able to do very advanced neurological stuff, like reverting the vision upside down, deleting specific memories, making intelligence random, losing feeling in arms - could not do such a thing?