You can find the lore in the rulebook (starting p.35)
As a relic of the origins of the Warhammer lore, Blood Bowl doesn't care much about countries and diplomacy. This is a game about iconic fantasy races hitting each other in the face.
So most events are related to Blood Bowl itself, its rules, its teams and its players, often under the form of anecdotes and quotes.
Just like Warhammer Battles used to be, it's sometimes quite inconsistent, which allowed the use of fun races or gimmicks which mostly disappeared from Warhammer Battles, like halflings, chainsaws and a whole lot of anachronisms.
A good testimony of this evolution is the unwillingness of GW to allow the Slann team in BB because it goes against the current design of that race. In the same way, Amazons used to have a Unicorn BG and you could also decapitate players with your chainsaw, play with their head, and score with it to get a RR.
Blood Bowl's lore and rules have become more standardised with time and the success of a more "serious" approach to the game, but its tier system and some specific players or starplayers are testimonies of a much more chaotic past.
This is way it is often considered that in the world of Blood Bowl, there's only Blood Bowl and what matters are only things directly related to BB. BB can cause epidemics, wars, massacres etc. but not the other way around.