Chaos in the Old World: What happens to units in ruined regions?
Nothing special happens. The figures just stay there and can be moved to another region in the summoning phase, or by other means. To quote from the rulebook, page 25, "Ruined Regions":


  When a region is ruined [...] the following rules affect that region:
  
  • New Chaos cards cannot be played to that region.
  • New Old World tokens cannot be placed in that region (although existing Old World tokens are not necessarily removed at the time of ruination).
  • During the corruption phase, no victory points are scored for dominating that region (i.e., domination is not checked for that region).
  • During the corruption phase, no corruption tokens are placed in that region.
  
  All other normal rules for regions remain in effect for ruined regions. Battle still occurs in a ruined region each round [...]


(my emphasis)