I've just re-read the "reading" of the Decks of Dragons by Fiddler in Letheras (in Dust of Dreams):
The cards forcefully pin the attendants to the walls, breaking some ribs and other bones. Throughout the city some power is felt by the elder gods like Mael and the Errant. Two gods helping Mael protect Seren even just burn and melt.
So either he has some power over the deck or the deck itself is more powerful than others we've seen.
Later on it is written that Fiddler managed to hold a few cards back.
So it seems it is Fiddler who can restrain the deck if he wants to. Which begs the question: Is he just another ascendant (who maybe even doesn't know it) or why are his readings so powerful?