When it enters, it tutors for any creature with toughness 2 or less
Dark Mist is also portrayed as an antagonist in the anime, and aims to destroy the world, as well as obtain the other Numbers so that he can rule over them as the strongest Number
All these factors combined result in a cards total value and how much players are willing to buy and sell it for
You have a better chance of getting cards from that archetype, so it's basically a green light to go for it
My main experience of Ugin is that he has to -X when he comes down and usually for at least 4 which leaves him incredibly vulnerable to burn, man lands, haste dorks and so on
Theyre both hugely popular characters even now, more than a decade after the first film was released