Demon: Hecatonchires
Requirements: Kymopoleia's Hair Pin</strong>
Go into a Suginami Tunnels Gold dungeon instance, and reach the Yagiya inside. Talk to him about a secret item, and end the conversation twice, in order to buy the Kymopoleia's Hair Pin at 12,000 iconi270001a.png .
Head to the field Shinjuku once you have Kymopoleia's Hair Pin, and activate the Hair Pin by double-clicking on it. A Field Boss version of Hecatonchires will appear, and you will have to defeat him. Take the Abyss Seed if he drops it.
With the Abyss Seed, head back to the Yagiya in Suginami Gold, and choose the option to show the seed to him. in exchange for the Abyss Seed, the Yagiya will give you the Hecatonchires Plugin: Eternal Hell.