We know that Void Century Elbaf has left a treasure box in the Owl Library that it is currently considered as a way to protect the country against Imu and his God’s Knights. Adding to that the fact that Imu has just said that there is ancient technology in Elbaf that he wishes to obtain, we can assume that this box contains this ancient technology.
What could this technology be? If this technology is supposed to protect Elbaf from Imu (!!!) and considering the current plot situation with Imu about to head back to Marigeoise and Luffy losing its will to its Zoan awakening, then my guess is that it is a weapon that cancels DF technology. Maybe a weapon that emits ocean soundwave frequency and which is able to destabilize a whole island area. My guess is that we will see it being used in Elbaf in order to save Elbaf from Black Nika and to bring back Luffy to his senses.
In my opinion, if I had to guess, it possibly isn't a weapon, I don't think there will be anymore Ancient Weapons around besides Pluton, Uranus (WG have it) and Poseidon. I think this may be the key to Pluton or the thing needed to activate it.
Lilly is from Alabasta, she has connections to Elbaph, also has connections to Wano as they created Poneglyphs and she played a vital role in them being spread all over the world which is why the WG are in the position they're in. I think given how Joyboy and co. believe in fate, they wouldn't put everything in One Place (e.g. a Treasure chest & Key), they'd put them in different places in hopes that the person meant to find it can form an alliance and trust from all the separate parties involved to prove they're really trust worthy.
I think this is the Key to Pluton or the Ignition tool in Elbaph, and Pluton is in Wano which we know - Biblo is probably giving them this tool to escape in case Imu defeats Elbaph.
If this was an Ancient Weapon which Harald was aware of, I'm sure the giants rulers before him would have used it to rule the world since they're warriors and only respect power. That is my impression but hopefully we get some more info soon.