A special sword that benefits from cleaving skulls at its owner's command.
"Aye, lad, the Great Tree- its got a branch in each world, and each branch is a world. If one were to find the place this world, this branch, met the main trunk, like I did, they could go to them other worlds. 'Course, you'd be some kinda poison, or, or, disease to the Tree, and that damn tree got one hell of an immune system. When me an' me buddies entered her trunk, she put up one hell of a fight. We fought for hours through every protection system she had, and all to get lost is this world fulla strange elves who glamoured us, trying to keep us as pets. We barely got back." -Old Gerald, man in the pub
Borrowed shamelessly from Norse mythology (see Yggdrassil, the World Tree), the Great Tree both connects and is worlds.
The great Wall Mounts have been scaled, or your ship's technology has reached a level in which it is possible to cross the oceans. And when the PCs or NPCs find other people, unknown people, what happens? Carnage.
The sun rises over the city. The great skyscraper's silhouettes from the fresh beams appear almost golden. The city is waking up, with the morning's half asleep citizens going about their daily routines. Starbucks and Dunkin Donuts open up, and revel in the influx of business. People head to work. The clocktower chimes, signaling the time of 9:00. But then a great shadow blocks out the sun. And the citizens of this metropolis start to scream as they are lifted up bodily by these beasts. An hour full of terror and screams goes by. And then another. And when the clocktower chimes again, signaling the time of 11:00, no one heard it. They had all been carried away. There were no flaming wrecks, nor collapsed buildings, as any other giant monster would leave. Simply empty streets, and a forbidding silence.
"Your wall is impenetrable, eh? Its so thick it can withstand 100s of direct hits from any siege engine? I have but one word for you: magic."