If you have any questions about Latin phrases or would like to make one up for your mod, I will answer any questions you may have. I have studied the classics for 12 years and am now writing my master's thesis on a Latin subject.
For starters, the plural of casus belli is casus belli.
Casus, plural casus: case, opportunity
Belli (genitive): of war
Since belli is here a common noun, it doesn't change either; the plural of casus is casus.
For starters, the plural of casus belli is casus belli.
Casus, plural casus: case, opportunity
Belli (genitive): of war
Since belli is here a common noun, it doesn't change either; the plural of casus is casus.
Code:
nom. casus pl. casus
gen. casus pl. casuum
dat. casui pl. casibus
acc. casum pl. casus
abl. casu pl. casibus