When the Lower Realm was in turmoil, an ancient ancestor-tier being had made a move from the Underworld, that was a Chaotic Ancient Corpse, projecting a clone, presumably the lord of the Underworld, who was very powerful and could summon the terrifying River of Underworld with a single wave of his hand, sending millions of souls to their doom. Nevertheless, he was easily defeated by the Willow God, and his reputed chaotic body was penetrated by the golden willow twig, leaving him utterly defeated.
However, even though the Underworld's lord was very powerful, with a Chaotic Aura swirling around him, if Jiang Hui remembered correctly, he wasn't actually the strongest entity in the Underworld.
The most powerful entity in the Underworld was a mysterious old man hidden in its depths who possessed the power of the Supreme Realm, possibly a decayed corpse from the Ancient Immortal Era, but he had been secluding since and wouldn't show himself easily.