The Vernal Pools Sphynx

Since the Divine Era when the gods walked among men, the Sphynx has kept it's home at the Vernal Pools. As a celestial being, they were brought along to the material plane as an ally to help shepherd humanity after the Scourge. They kept their distance from humanity, being unsure how to interact with them, and preferring more to learn of them from a distance and combat any great threats that popped up in their region. When the Dividing Barrier began construction, the sphynx was asked to stay behind - not being a god it would be permitted (this could raise the question of who else was permitted to stay on the material plane - angels? Lanfir?). After so much time it has grown rather resentful of this decision, as it now lives a lonely, solitary life, separated from other sphynxes and celestials, though unable to integrate with humanity, as most people in the past either feared it or revered it so much that they couldn't hold a comfortable relationship. They harbor some ill will towards dragonkind, who they consider unrightfully ingratiated with the gods via the war against the Umbrals.
The Sphinx guards a collection of scripts of Deneir, an aetherium shard, along with knowledge of key celestial locations on the Material Plane. One script he had given to Godfrey (the secret cultist who disappeared from the Caverene tomb) after Godfrey proved to pass his tests with great feats. The Sphinx still keeps in their custody the script detailing the events of the Scourge, one of Denier's most important records.
The Sphinx in the trial will call upon angelic echoes to fight alongside it, which will summon out of the runes drawn on the ground. These are temporary ethereal creatures, just shadows of angels that the Sphinx knew. They use the Deva statblock, except they only have 50 HP and will disappear if the runic circles are broken. Solus (alone) is the name of the Sphinx, though they used to be known as Sophos (wise). They will summon the following three Devas, or memories of them pulled from pocket dimensions. (names and inspiration from https://www.dndspeak.com/2018/04/27/100-angels-and-their-descriptions/)

• Eska, Angel of Lost Time, whose kiss can unpetrify one and then transfer the petrification, wielding a great mace
• Rashgana, wielding a great hammer and AC 20 in glimmering plate armor
• L'Araine, tall woman with lion headed shield AC 20 as a weapon

The following information is summarized in the script about the Scourge in the Sphinx's care: Lyir destroys what was the Emerald Isle, shattering it, sinks the great city of Ashanda (in the Bay of Rias). It is not until years later that Lyir is tracked down and restored by his wife Umberlee, but that causes her corruption in return. Bhaal (murder) infiltrated near Haven and urged men to murder, in an event we're calling the Hilltop Massacres. (Bhaal then fled underground, but Lathander's wrath found him, and plummeting to the earth to draw him out, the Shattered Steps were born, and Lathander killed Bhaal)
Cavere fell to manipulations and corruption by Bane (tyranny) and Cyric (lies), who urged the queen to cast all nonhumans over the cliffs and into the sea (this was before even Queen Pradesh). Tymora and Tyr saw they were brought to justice and cast out of the city. The Pantheon pair landed at the Pebbled Shore to establish their camp in what are now the ruins of Corasus, before marching to take back Cavere from the Lanfir and devils within. (Tyr is now being corrupted by Cyric)
Talona (disease and poison) destroyed settlements at the base of the peninsula, filling the earth and waters with acid, wilting the trees, and making the air too noxious to breathe. Talona fled after doing her damage, not one for combat, and Mielikki tried to restore as much as she could, but the sick took root)
Malar (evil hunt) and Loviatar (pain) travelled along the mountain range on a murderous rampage, until stopped by Selune and her silver chords, though the place where Loviatar was struck down and wounded, letting loose a pained shriek that shattered the mountains in modern day Loviatar's Maw. Malar esacped and fled.
Beshaba (misfortune) and Myrkul (evil death) together buried the great city of Ishtana. (modern Wastelands - in the Underdark lies the root of this destruction, guarded by a squad of Lanfir who are frozen in a timestop by Torm, who had to sacrifice himself in frozen time too, in the palace known as Ihliziyo, once the center of Ishtana, Torm and 6 Lanfir lie frozen in Ihliziyo).