This article is about a phenomenon often called the Disappearance of the Sword of Fury. It's also called the Dungeon Floor Secret, and the authors of such a name are game creators themselves, and it started long ago when the trick was still in the making.

In order for Avar Tar's sword to disappear, we have to step on two specific tiles located on Rookgaard, in exact order.

Firstly, we have to go to the Poison Spider Cave and enter a hole with three poison spiders near Captain Iglue's Treasure Quest and stand on the square shown in the picture below.
Next, on your way to the Katana Room Quest, you have to visit Rotworm Hell and step on the tile shown below.
If you go to see the Spike Sword, you will notice that it is no longer there. There have been multiple theories concerning what happens next with the blade. Multiple adventurers tried fishing the sword up from the nearby waters or from the waterfall underneath, fighting the lighthouse keeping minotaur unequipped, or opening the Minotaur Mage's room, some even played with the arrangement of bodies at the altar near Wasp Tower, but with no effect.
The most prevalent seeker was Pan Klocek.

On a final note, the available sources state that the sword never appears on the island after the disappearance, which means that the rumor of an unfinished or deactivated quest seems extremely likely.