“It weighs nothing. The ground beneath it disagrees.”
The Hollow Fragment is the offering used to summon the Harrower. It is an irregular, dark stone fragment with a pale fracture running through it.
What it does
The Fragment is not a weapon or an ingredient in the Hollowforged crafting recipes. Its purpose is to activate a correctly constructed Hollow Cairn.
Drop the Fragment as an item into either of the two open cells in the Cairn's center. The ritual consumes one Fragment when the summoning succeeds, and the Harrower arrives nearby.
Obtaining a Fragment
Hollow Fragments are intended to be discovered through exploration and loot rather than crafted at a crafting table. Look in top-tier chest throughout all vanilla dimensions.
Can I reuse it?
A successful summoning consumes one Fragment. The Cairn itself is reusable, so you can perform another ritual when you find another Fragment.
For the exact construction layout and ritual, see Summoning the Harrower.