I don't like it when Wizards prints a commander that just says "here is the best commander for X archetype
This doesnt force them to attack like Goblin Assault, doesnt make 0/1 creatures like Awakening Zone and only has 1 colored mana symbol
You've even read through the recent Magic Story, so you're up to date on what's been going on to bring us to this point
But with the right pieces Cosima shines in vehicle decks, quirky landfall decks like Tameshi, Reality Architect, and even in exile-matters decks like Ranar, the Ever-Watchful
Now, Grim Flayer doesnt have the stats to get there by himself, but if the deck is built around him correctly, then he can really shine
But the upside of that turn-1 Necro, or using this to fuel a long turn that ends in Tendrils of Agony, more than makes up for that weakness