Tiamat

Cast her, and the god pays out: up to five Dragons, each with a different name, searched out of your entire library and delivered straight to hand at no further cost. That is what the full five-color pip tax buys. The queen of evil dragons is not a threat you win with directly; she is a threat you assemble around, a one-card refuel that guarantees a Dragon deck never runs dry the moment she resolves. The build discipline lives in the "different names" clause: a toolbox of singletons beats a stack of redundant beaters, and the more varied and interlocking your hoard, the bigger the haul she hands you. Note the exact trigger condition, too. The tutor fires only if you cast her, which cordons off the shortcut lines: reanimating her, blinking her, or otherwise cheating her onto the battlefield leaves you with a 7/7 flier and a dead trigger. Crucially, "cast it" covers the command zone as much as the hand, so paying the honest cost, wherever she starts, is the only way to collect. Nobody sinks seven mana across every color for the combat math a large evasive body provides; they sink it because searching out an entire game plan in one spell is about as close as Dragon tribal has come to drawing its whole hoard in a single resolution, then daring the table to outlast what all those Dragons do next.