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Marrow-Gnawer sac-loop: tap Marrow-Gnawer, sacrifice one Rat, make a token for every Rat you control then swing with fear for a one-sided alpha strike

It only uses cards with the true partner mechanicno partner with, backgrounds, or any of the other partner offshoots

First Edition cards have a small "Edition 1" oval stamp printed on the card

See here for a full deck guide on Edgar Markov

In the case where Fealty to the Realm is enchanting a creature but there is no monarch, the second ability will create a control-changing effect with the timestamp mentioned above, but that effect won't do anything until a player becomes the monarch

Yes, you can counter a triggered ability with specific cards that say counter target triggered ability. Regular counterspells like Counterspell only work on spells, not abilities already on the stack