We don’t know how exactly Leviathan was involved in the incident, but we do know she’s trying to kill a former king of a kingdom she belongs to. Note that it isn’t specified if the kingdom Leviathan is from is still around, so keep that in mind.
We may not have encountered all the factions yet, but I’m gonna list the kingdoms I think Leviathan could be from
List of potential Kingdoms
The Grand Navy/Astoria
Not Likely, but could be more likely if the incident Leviathan was involved in was related to what Commodore Kai hints at when killed, although this entirely depends on Leviathan being former member of the GravyRyujin Dynasty
It's probably a stretch since it combines aspects of 2 different mythologies. Leviathan is the name of a sea serpent demon in the Hebrew Bible. In Japanese folklore, the Umibōzu is a yokai that has a tendancy of causing turbulent seas and destroying ships. Another thing the Umibōzu is capable of appearing without disturbing the water and cause a storm to appear when seen. In the Bakemono Chakutōchō, there is a painting that depicts the Umibōzu as a large fishlike humanoid. It's a massive stretch to connect Leviathan, a sea serpent, to a single depiction of the Umibōzu with the commonality of scalesTldr: Leviathan is a sea serpent, the Umibōzu can cause storms and has been depicted like a fish, both have scales, so Umibōzu could be a "leviathan" of the sea
Thorne Empire
Unlikely. If the incident Leviathan was involved in may be referring to the reason why the Thorne Empire left Cassia Town out of fear and severe losses, the only question I have would be why leviathan is going after the former king for thisVisigoths
Higher possibility depending on how long ago the mass sacrifices were on Makrinaos. It can take up to a few weeks to several years for body to skeletonize depending on the factors. Leviathan being a survivor of this incident would make her target the banished king that's in the Abyss of Charon. Wanting revenge on the banished king is a valid reason to set out to sea to get stronger, especially if the Visigoths are still present in the War Seas. The only downside I could think of in those proposal is Leviathan appearing in any sea after the nimbus sea, unless the banished king is meant to be on the scale of jrpg super bosses.Anyways, where you think Leviathan is from?