Creatures that enter attacking are never declared as attackers, and as such, they won't cause Demonic Covenant's first ability to trigger
Stealing something like an opponents commander or combo piece can absolutely shut down their game plan, though you have to be careful since it is an aggressively unfun strategy
These are essentially the 1st Edition cards of Disney Lorcana
Pile shuffling has several additional benefits as well: It runs less of a risk of the player accidentally seeing a card or dropping a card face up, since they can be held with the faces pointing straight downwards, unlike mash shuffling where that would be an extremely awkward position for one's hands
Finally, new tournament dates were confirmed for the upcoming American and European championships
The main ones you're looking for are the 'Karn-struct' Construct tokens that get +1/+1 for each artifact you control