Scripts for beginners.

Look at the hero's script name in its settings (Select => Space => Settings => Script name). And replace "hero" with this name. That is:

function Def( heroname )
if heroname == hero then
StartCombat(hero,nil,1,CREATURE_CRYSTAL_DRAGON,6);
Trigger( REGION_ENTER_AND_STOP_TRIGGER, "def", nil );
end;
end;
Trigger( REGION_ENTER_AND_STOP_TRIGGER, "def", "Def" );

Since you left the last 4 parameters as nil, you can simply omit them.