The Artifact of the Grave of the Unknown Hero in the "Mogila of the Nameless Hero" is not simple. It is not enough to just go to the hero and interact with the grave, you need to get the curse. It can be removed by taking the curse or by simply walking past it. The curse lasts usually no more than a day. You can see the exact duration and the power of the curse in the character screen in the left part of the screen, you just need to hover over the cursor.