The Beneath is one of the largest demon planes that has the strongest connection to the plane of Jaern. It appears the demons of Jaern are native and come from this plane. There are no known ways to enter The Beneath, but increased demonic activity have revealed some knowledge about this place to adventurers.
There have been reports of an adventuring party known as The Chained Leviathan who seems to be involved with something concerning The Beneath, but little is known of their motives. Other adventurers1) have been sent to investigate on behalf of the Xurora Assembly in Tephis, Ageron to investigate their movements and the ruins of Belthega…
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The Beneath is described in this Google document.