Once you all of Sir Zombie Brains'es quests, and play for 30 minutes after, Sir Zombie Brains runs from the bottom of the screen and says "*Player*! Something terrible has happened! Follow me!" and you will get the choice to follow Sir Zombie Brains. If you do, he takes you to a fake Root that is demolished and with NPCes sheltering. He says: "A nearby Dragon was disturbed by our adventurer Zone115. He flew to Root and destroyed it! We need you to venture into the mountains and slay the dragon. Here, you'll need this." You receive a Legendary Sword that is only able to be used in the Storyline, and cannot be traded or placed or sold in commands or in buyboxes. "Use that sword to kill the dragon. We think he is nesting in the mountains on Grassy Doom Peak."
If the player travels to Grassy Doom Peak, he sees the Dragon. If the player walks towards the dragon, it will slowly rise up, and if the player goes too close it will turn around and roar. The player needs to then solve a maze of fallen rocks due to the vibrations of the roar. If the player succeeds in less than a minute, then the player goes and strikes the dragon. He then flies away. The player will automatically go back to the Broken Root.
Sir Zombie Brains will congratulate you and give you a Vial of Poison. This is a unique item and if thrown on a pile of enemies, it will poison them making them more vulnerable and giving 2 and a half times more gold. Alternatively, Sir Zombie Brains may give you a Teleportation Rune, which can be used infinite times(3 times per day, though) which zooms out and lets the player teleport anywhere within 300 blocks and in the same Universe. Sir Zombie Brains immediately has a new quest. In this quest, you need to go underground and slay the dragon once and for all. Once you go underground and meet the dragon, he slays you with his powerful teeth before you could strike him.
After being slain by the Dragon, you go back to the Broken Root all beaten up. Sir Zombie Brains gives you the sword again, and sends you back underground to try again, this time with another poison vial.
When you go back to the Undergrounds, a NPC named Kralereth calls from behind you, "Throw the poison vial and strike him from behind!". If the player does what Kralereth says, then the dragon will be slain.
Sir Zombie Brains congratulates you, and says you need to go to the Volcano to slay demons that have been terrorizing the people of Root for years and years.
I think you can come up with more after that.