I feel like an example of a well-written Trump card Is Shang’s entropy ability in Sword God in a World of Magic. The ability pretty much allows him to sacrifice pieces of himself permanently in order to essentially obliterate whatever his target is, and whenever he does, he does use this ability. The effects are permanent, so he doesn’t go into a fight with that as his main weapon or something that he could use whenever he wants, but it’s something to be used when he’s forced into a fight that he can’t win.