Variables
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Unitcount - Integer - Init=0
Trigger 1
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Events: unit enters region 001
condition: (define what unit should enter, should it be owned by player 1, should it only be a footman, or ...)
Action: Set variable: Unitcount = Unitcount + 1
Trigger 2
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Events: unit leaves region 001
condition: (define what unit should enter, should it be owned by player 1, should it only be a footman, or ...)
Action: Set variable: Unitcount = Unitcount - 1
Trigger 3
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Events: unit enters region 001
Condition: Integer: Unitcount >=5
Action: Pick every unit in (units in region [owned by player 1 if that was a condition in trigger 1 and 2]) and do action: kill picked unit
Unit - Create Thenewunit at center of region 001
If this should happen every time you have 5 units in your region, add the following action to the last trigger
Set variable: Unitcount = 0
If this should not happen every time, I'd suggest to add the following action to the 1st and 2nd rtiger:
Trigger : Turn off trigger 1 (and 2)