Leviathans and Beasts
To the Beasts Leviathans are the biggest proof of the veracity of their claims since not only share a common origin as children of the Mother of Monsters, Tiamat, but they also can see Marduk as the First Hero. The fact that they also create Heroes(The Ahabs) is considered concrete evidence, so in general they get along very well. However, some consider them failed prototypes because, from their point of view at least, they need to Feed all the time and their human cultist creeps them because brainwashing people to serve you didn't teach them anything. Also there is some envy because they can put their real form in the human world without apparent problems and can breed true, something most Beast and many more monsters, except maybe Changelings, would like to be able to, and, also, they would like to get into the Tempest because they believe it can be the Primeval Dream.To the Leviathans the Beast are descendants of Progenitors whose names they can't recall, so they return the sentiment but would also like to have their Hungers because is less sucky compared with their localized mindrape. Also, they get along better with the Makara than the other families and their "cousins", since both share the archetype of sea monsters.
Lemuria and The Seers Of The Throne don't know about each other because of the Marduk Society
They have hands in both organizations, so it makes sense. of course, if the two were to meet they would go to war. both organizations have reason to fight leviathans on a basic level(leviathans are a high level threat) and produced despair for the dark sorcerers secretly running things. If they wiped each other out, the society would have less despair and less organizations to get stolen Intel from under the radar.