Just to confirm the surmise of stevesliva and aparente001, a Google Books search finds this quotation from by SamanthaBlackWhitlock, "The Potter Twins and the Goblet of Fire" on FanFiction.net:
On the landing, Frank turned right, and saw at once where the intruders were: At the very end of the passage a door stood ajar, and a flickering light shone through the gap, casting a long sliver of gold across the black floor. Frank edged closer and closer, grasping his walking stick firmly. Several feet from the entrance, he was able to see a narrow slice of the room beyond.
For some reason the identical wording appears in a snippet view from what is reported to be Newsweek, volume 136 (2000), page 70.
But DPhantomtomboy, "Danny Phantom and the Goblet of Fire" (July 5, 2006; updated July 10, 2008), also on FanFiction.net, has this:
On the landing, he turned right, and saw at once where the intruders where: At the very end of the passage was a door that stood ajar, and the flickering light shone through the gap, casting a long silver of gold across the black floor. He advanced closer slowly, grasping his walking stick firmly, as he was several feet from the door he could see a narrow slice into the room before him, the fire.
As you can see, DPhantomtomboy has considerably more trouble with spelling and syntax than SamanthaBlackWhitlock does. The OP's quotation above matches SamanthaBlackWhitlock's sentence except for following DPhantomtomboy in using silver instead of sliver:
At the very end of the passage, a door stood ajar, and a flickering light shone through the gap, casting a long silver of gold across the black floor.
My conclusion: The word silver is supposed to be sliver in the OP's quotation. But the source of the original wording (presumably somewhere in the J.K. Rowling oeuvre) and, in general, the mechanics of fan fiction versioning are mysteries to me.