Sterling, Voh, and Adran were sitting around in the Burning Man Inn, drinking and remembering the good times. You know, before the Gods started hating them and they could move around in peace. Then they were approached by two bald-shaved humans, servants of some mysterious master who wished to meet them. When they mentioned they would pay for their drinks, they agreed.
So later they met with this mysterious person. They were again at the inn, and there entered a humanoid figure clad fully in some sort of rubber/leather suit, covered head to toe to grant him his own private atmosphere. But later his servants removed his oddly-shaped helmet to reveal a face of pink skin, milky white eyes, and a cluster of tentacles stirring where you'd expect a mouth - this fellow was a mind flayer!
But it turns out he's on their side. The illithids wish to see the entity known as Gizibbemuthor born into the world. And for that it is best if the PCs are alive. And with their many ESP abilities, clairvoyants, and prophets, they had learned an important secret: the Gods can't reach the PCs while they're in Sigil. But they have recruited the help of an evil cabal of chronomancers to assassinate the PCs in the past!
Fortunately there also exists a counter-faction, Time Lords if you will, who strive to keep the timeline unpolluted by mortal interference. An agent of this faction will contact them soon. Since they are already dead in the past, there is a wave of causality rushing forward through time to erase their presences. They need to move quickly and accept the offer of this agent, for there is no time to lose! (Pun not intended.)
And indeed, the next day a Mysterious Cloaked Figure approached them. As she pulled back her hood, she revealed herself to be a woman named Vu.
(And this requires some background. In a campaign I play in, I play Deja. She's a shaman refluffed as a chronomancer. Her spirit companion is called Vu, and she's not a real spirit, she's a temporal echo from an alternative timeline. And now, in a different D&D campaign, we meet Vu. And you can probably guess what her own spirit companion/echo is called.)
Anyway, she told them what they already knew. Back in time to kill them, evil chronomancers, come with me if you want to live, that kind of thing. She told them that the first step is to identify the exact point of time where the assassination(s) occur. Time travelers tend to keep meticulous paperwork around, because there's always a lot of risk to run into other time travelers. And that's no good: if a timeline is altered more than once, it gets highly resistant to further change. It "ossifies" as it were. Which is good new for Vu and the Reapers: if they can prevent their assassinations, their own pasts will become "locked" and it can't be attempted a second time.
And so they sought out a portal which Vu manipulated to bring them to an odd demi-plane, where a grand structure was shifted away into Just A Second Ago, there at that point in space but always hidden in terms of time, inaccessible to normal people. There they met the gate keepers: one Fomorian Portal Lord, two quasi-undead soldiers whose lifelines were frozen in temportal stasis, and three odd spherical creaturres who were all gnashing mouth and teeth... they were Langoliers!
But the PCs kicked their asses. And now they move forward into the structure, which resembles a temple-palace built in a mish-mash of every architectural style that ever was and will ever be. What will they find inside...?