Soul rune

Soul Rune is a members-only item on the OSRS Grand Exchange in the runes category.

In-game examine text: "Used for high level curse spells."

It has a Grand Exchange buy limit of 25,000 every four hours, a high alchemy value of 180 gp, and item ID 566.

View on OSRS Wiki

Soul Rune shows an active flip setup with a margin of +3 gp and 23,362,408 trades over the last 24 hours.

At the buy limit of 25,000 every four hours, max profit per cycle is about 75.0k gp — high margin with healthy liquidity.