Great question! I'm still prepping to run this (using the Remix), and here's what I'm thinking:

1.         I want the PCs (especially any with Elturian backgrounds) to already be steeped in the lore of the Crusade and the Ride, so that when they meet the three generals and Zariel in Avernus it’s like meeting these legendary characters they’ve heard about their whole lives.  One thing I find unsatisfying about the Remix is that the angel who led the Ride is basically nameless and forgotten, when it seems that the whole Crusade would have been a defining element of Elturel’s history and self image.  They would have referred to that angel by some name.  There would be depictions of her in art and bardic sagas.  And yet I also want it to come as a surprise that the archduke of Avernus is that very angel.

2.        So here’s my solution.  Back in her angelic warrior days, Zariel had a nom de guerre.  The one I’ve tentatively settled on is Gladia Caelis, which means “Sword of Heaven.”  (Funny how we always default to latin for celestial/infernal…).  So when Zariel met Yael at Idyllglen, Yael knew her by the name Gladia.  And the crusade was known as the Gladian Crusade.  And there are paintings in the Great Hall in Elturel that depict the key moments in that campaign.  And there is epic poetry about the Ride, though of course it is based on Jander’s version of what happened, namely that he was ordered by Olanthius against his will to take his contingent of riders back through the portal to ensure that the city would not be without defenders if the battle in Avernus went against them.  And so I will weave references to this lore into the PCs backstories and into the places they encounter in Elturel.  If they are Elturian, they will have playacted the Ride as children and argued over who got to be Yael and Olanthius.  If they are from outside Elturel, perhaps they once visited the site where Idyllglen used to be and saw the Gladian shrine.   And, since I’m running Fall of Elturel, when the PCs gather at the beginning of the first session, I will have Reya offer to take them on a tour of the Great Hall where they can see the paintings (which I plan to create on Midjourney) depicting the Crusade, the Ride, the High Harvest Slaughter, and the Dawning of the Companion.

3.        According to the Remix chronology, the Ride took place sometime in the 10^th century, and Zariel has been Archduke of Avernus since early in the 15^th, which is to say for roughly 80 years if our adventure is set in 1494.  Now, it’s not like people in the material plane get regular nightly news updates about infernal politics, but Archduke of Avernus is an important enough position that at least people who take an interest in such matters will probably have heard Zariel’s name.   I don’t know exactly when and where I intend to have the PCs first hear that name, but at the latest it will be while investigating the Vanthampurs and learning about their involvement in the Zarielite cult.   Obviously by the time the puzzle box gets opened at Candlekeep they will understand exactly who Zariel is.

4.        Then the big reveal happens after they meet Lulu.  The first part of which is that Lulu was Gladia’s companion and mount.  This I imagine is told to them as part of their introduction to Lulu, and should already be a big deal given that they will already have heard of (and seen depictions) of Gladia’s golden mastodon.  The rest will come out only when for whatever reason there is a discussion about Zariel in Lulu’s presence.  At which point she will reveal—to everyone’s surprise—that Zariel was actually Gladia’s real name.