Since a few years Sigil-time, more and more talk about a new Abyssal lord comes from chant-mongers in the Cage and elsewhere. Most just call him "The Great Lord", and the darks told usually contradict each other heavily - yet it's possible they are all true.
Rumours have it The Great Lord is not a "normal" Abyssal Lord (if you can consider any Abyssal Lord "normal"), but the combined belief of a whole bunch of Abyssal Princes, rulers of palaces and fortresses in the Abyss' first layer.
It's told these princes made a plan to work together on acquiring Lordhood. Each of them gives a part of his essence and power into the newly created creature, and with all their power combined, The Great Lord is mighty enough to compete with any of the "real" Lords.
As more and more stories of The Great Lord come up, some even speaking of an avatar walking the Abyss, it seems there's some truth about the rumours.
The existence of such a creature could have strong effects on the entire Outer Planes: The princes are residents of the first layer, and so The Great Lord is assumed to reside there, too. If so, it's entirely possible he succeeds in making the entire first layer his realm - and with that, he'd probably be the most powerful blood in the whole Abyss, controlling most of the portals into and out of this plane.
It's also unclear whether the princes truly work together, or, far more probable, scheme against one another, each of them trying to gain the entire control over The Great Lord. He himself, on the other side, might be out to break free of ANY control, becoming a free creature of its own - or maybe even swallowing the souls and essences of all the princes that created him.
The other Lords seem to be confused about the new presence. Few darks are spilled, but the bloods among the chant-mongers tell that The Great Lord is somehow "uncatchable" - a creature that is real and unreal at the same time, existing yet not part of this existence (yet).
Some even say that a few Lords try to support him, either to gain something from an alliance, or to fully anchor him in the Abyssal reality, and afterwards being able to attack him like any other Lord.
So far, though, The Great Lord's presence is felt by few mortals and only slightly more tanar'ri. Most who tell about him say they've met his Avatar personally, or a servant of one of the princes that created the Lord. The Avatar is described as being a giant, beautiful bird engulfed in golden fire. Its behaviour and speech as well as his eyes seem to change constantly, though - most probably these things adapt to the prince who is currently in control, or it flips constantly as the princes are all constantly in control.
One of the bigger darks told about The Great Lord is that he's currently on a mission: Some residents tell they've observed him looking for the soul of a specific creature, a tanar'ri called Ju-Shair-Muth. They tell Ju' has been an Abyssal Lord millennia ago, but failed in keeping control of his realm. But as the layer tried to swallow his soul, he escaped, and sleeps since then, recovering his strength.
What The Great Lord might want from Ju-Shair-Muth is hard to guess, and since these stories are all told by tanar'ri, it's actually a big question if there's any truth about it at all.