When a monster's effect is activated, while a monster(s) with its same name is in the GY(s) (Quick Effect): You can negate the activation
When you compare this to other options, like Goldspan Dragon or Stormbreath Dragon, it just feels a little meh
Thats not to say that the other versions of Charizard across the Pokmon TCG dont have value though, as several of them do
Dimir Spybug does this same thing, though the idea of more surveillance leading to more spybugs being requisitioned seems a stronger link
A word salad of broken abilities and effects, this Ulamog, along with his Eldrazi brethren Emrakul and Kozilek, set the stage for three of the strongest characters, canonically, in MTG history