Some races, specifically Skaven, Dark Elves or Hobgoblins, are so untrustworthy that not even their own kin can trust them for long. Worse, their manipulative presence tends to sour relations between other allied parties. An alliance that includes Skaven, Dark Elves or Hobgoblins is automatically a fragile alliance.
All relations in a fragile alliance start out normally, according to the various alignments. However, each player in a fragile alliance must roll a D6 at the start of their turn – on a roll of 1-3, all of that player's alliances immediately become one step worse for the rest of the game (unless they're already desperate allies).