A Seriglass Shard is a resource that can be purchased from Grandmother in the Necralisk and is used to craft Entrati weapons. It is also required as a sacrifice to attain Family rank with the Entrati.
“You're not supposed to be in here! You're going to ruin the surprise!”The following section contains spoilers.
The Seriglass Shards appear to be the shattered pieces of Albrecht Entrati's Seriglass Bell, which shattered into fragments upon his first voyage into the Void. This is supported by the item's description stating it was once part of a larger apparatus. Upon waking on the floor of his laboratory, Albrecht himself used one the broken pieces to write "Close it" (referring to the portal he had just opened) in his own blood. It seems that his heirs saved the pieces of the Seriglass Bell, as they can be obtained by the player from Grandmother, and Mother is mentioned by Father as keeping a shard for herself (and even using it as a weapon against him).