This setup had two problems:
A) How to technically deliver a reveal about something that happened in the distant past such that it's cool to read about. How to show something that essentially isn't there to see for about a million years.
B) Making it emotionally satisfying --> stakes.
A) Narrative techniques
Stories are about delivering information. The way you deliver it often makes the difference between how that information is received. After thinking long and hard, I realized there simply aren't that many narrative techniques to deliver information:
1) Exposition is always worst. Telling readers through narration any kind of information can be flat, dull, and unexciting, which leads to writing an infodump.
2) Dialog is the next step up. Mixing information in dialog is like putting spinach in the blender to feed it to your kiddy hidden in their fruit juice. If you manage to make a fun concoction, all good. If not, they're going to spit it back out.
3) Showing or dramatization is the enactment of the reveal through actions that unroll before the readers' eyes. This is almost always the best narrative technique (except specific cases).
Everything that can be communicated in a story reduces to seeing it, hearing about it (dialog), or being told a summary (exposition).
Here, I needed to reveal past action. There was stuff to show, not just ideas, but I couldn't directly show it because the action had already finished. Something bothered me about it, it felt disembodied and distant. I thought it's because the characters can't participate directly in it, i.e. see it. I thought upping my narrative game is enough and maybe it would've been but something still felt off. I was not satisfied.
A narrative technique like showing can't fix them. Though mine was a historical event, I found ways to dramatize it -- show it -- through something like a hologram, or some recording (these are not narrative techniques themselves but ways to implement the narrative technique of showing). It still didn't feel like it worked. I went one step further and came up with a minor technology that relays sensory information, i.e. make it "real", so I can pull readers into the scene with it. Something was still missing.
The biggest problem with something that happened in the past is that you can't stop it. You can't walk in on the bad guys, pull out a weapon, and demand them to stop. So then, what is at stake? What is the point of watching this reveal other than "it's cool" and "we waited the whole book to find it out"?
That's what felt off.
What can be at stake?
1) The long-time-ago reveal still influences the present (or it wouldn't matter - this was my problem, I wanted it to matter). You can't prevent it from happening, but you have reason to want to stop it now, reverse the damage, or you'll have consequences, die, the world ends etc.
2) In my case, the characters found out about the reveal in the first place because the magic had one of them seriously affected and they were due to die. And the other main cast cared, which amped stakes.
3) My reveal also implied that others will be soon affected too, if they weren't already, so there still was something to be stopped by tweaking the way the magic worked, even though the characters couldn't prevent the bad guys from altering the magic system a long-time-ago.
Last step: Tying it up
After setting up stakes and deciding on narrative delivery, the last step for creating an exciting reveal is to connect the reveal with its consequences and show real world impact to put it into perspective for readers. That's how the feeling I was trying to touch a ghost went away.