Ghorm Summoning Idolゴームの召喚像

The Underground is full of tunnels that were never dug by hands. Hold this idol close, and you can almost feel the ground moving.
In-game description

How to use Ghorm Summoning Idol

Ghorm Summoning Idol is a summoning item for Ghorm the Devourer. Use it to fight the boss again, for example to farm its drops.

Go to where Ghorm the Devourer is fought (its arena).
Place Ghorm Summoning Idol on the summoning rune there to make Ghorm the Devourer appear. The item is used up.

Most boss-summoning items are pieces of furniture that you place on the boss's summoning rune; the in-game description of the item says which rune. Where the boss is fought is on the Ghorm the Devourer page.

Ghorm Summoning Idol key points (from the game data)

Sells to merchants for 40 Ancient Coins.

Calculated and summarized by this site from the game data (1.3.0.6).

The game data has no recipe or source for this item.