Just like the other Glenumbra dungeon shards, this one also had its location changed. At this time, you can find it in the northwestern part of the dungeon. This is one of the shards found close to the entrance of the dungeon.
Entrance to Silumm is in the ruins to the left in the above screenshot.
Enter here to visit Silumm.
Next to the broken stone statue.
Exact location of the shard.