The Diamond Sword of Tor

Five hundred years ago, humans burst through a portal between realities bent on conquest. With the help of the Elves from this reality, they tried to eradicate the dragons. Things did not go well, and the invaders moved the Elves to safety between realities and withdrew. The power used to move the Elves backfired through the portal damaging the invaders' reality severely, and the dimensional key in the form of a Diamond sword was left behind. But now after five centuries the sword hidden behind a disguise of steel, has found its way into the hands of Erun Oncant, Sovereign King of Cardoney. A child at play caused the portal to allow a single team of humans through, and they need to recover the key to return home. When they don't find it in the castle they kidnap the Queen, to coerce Erun into returning it.