OK, we got a lot of information from Star and Marco’s Guide to Mastering Every Dimension, I’m just going to organize the lore stuff, for more I would highly suggest buying it for all other Misc infos (including mewni’s queens infos.)

1- there are two monster royal families or one monster royal family and one particular species of monsters:

1.1-Avarius (Ludo’s family, they are protected by the butterflies)

1.2-Septaris (Toffee and Rasticore species, capable of regenerating)

1.3-There is a treaty of peace (or alliance) between both, that was broken when castle Avarius (Ludo’s castle) was taken by toffee: Avarius-Septaris treaty

2- there was a conflict that led to an accord.This accord is the political status quo between mewmans and monsters: mewman\monster accord

3-There is not enough information to know what, treaty or accord, came first

SPECULATION

Avarius and Septaris have different ideologies:

1- Avrius seeks only peace, Septaris seeks equality or more.To stop internal conflicts they made Avarius-Septaris treaty

or both wanted the same thing, but they lost in the end:

2- Both groups wanted more influence over mewni and so was made the alliance, after it the conflict started and peace was only achieved at the mewman\monster accord.