Second of all, it's an artifact dork for those that need double nice
That sure is a lot of fantastical terms all at once, but essentially it means that were storytelling magicians competing against each other by summoning copies of Disney characters to see who can gather the required lore first
Medomai the Ageless and Rasputin Dreamweaver are both legendary creatures at 4UW that outclass Kasimir
Since the creature was put onto the battlefield attacking, it was never declared as an attacker
Some important things to consider about Marneus Calgar: Every creature that enters the battlefield and makes a token, Treasure, or Clue is effectively a cantrip
Drawing two cards and creating a Treasure token, while also being able to sacrifice an artifact instead of a creature, is just enough value to push it over the edge of other similar cards