The Shard of Pure Evil

The Obyriths, ancient entities of unfathomable evil from a decimated universe, once forged a Shard of Pure Evil, channeling all their hatred and insatiable hunger into this malevolent artifact. With the shard, they pierced the barriers between universes, traversing into a new plane of existence.
In the default D&D universe, the Shard of Pure Evil was discovered by Tharizdun, a being consumed by darkness and corruption. Instantly corrupted by its malevolence, Tharizdun wielded the shard's power to shape and create The Abyss, a realm of infinite chaos and torment.
Within The Abyss, the surviving Obyriths clashed with primordials, engaging in fierce battles that ultimately transformed them and the primordials into powerful demon lords. While only the Queen of Chaos, Pazuzu, Obox-ob, and Dagon are explicitly confirmed as Obyriths, their malevolent essence permeates The Abyss, shaping its very fabric.
As the Shard of Pure Evil delves deeper into The Abyss, it continues to carve new layers, feeding on the magical energies, souls, and matter of the surrounding planes. Its insidious presence within The Abyss serves as a catalyst for the twisted evolution and growth of the realm, forever expanding its depths and horrors.
In a separate note, it's worth mentioning that Asmodeus, the archduke of the Nine Hells, wields a powerful artifact known as The Ruby Rod. This unique item grants Asmodeus the ability to unleash devastating offensive and defensive spells. The Ruby Rod itself is believed to be a fragment of the Shard of Pure Evil that birthed The Abyss, providing Asmodeus with a connection to its dark and destructive power.