6 Cinderella, Stouthearted (Challenge Promo, 3) $160.50 Take On The Challenge Challenge promo cards are only obtainable by the best of the best as you have to work your way towards them by performing well in the official Challenge events that are held each year
This doesn't seem wholly sufficient to explain the spike, otherwise why didn't it pop off during Bloomburrow, which brought in a popular new bird commander Kastral, the Windcrested
Do note that Kotori gives any target artifact creature lifelink and vigilance
MTG also has its share of underrated cards, and if I know anything, its that Magic players are highly opinionated and never wrong about card evaluation
Bracket 4 (optimized) and 5 (cEDH) are unrestricted on game changers
They enjoyed how we conveyed the story through the cards