I've used this code before in my Trafalgar Square Wonder and as a UU ability in Thomas' War. This it just a modification to use it as a promotion.
By 'Plunders' I mean it removes the gold from your rival's treasury and places it in yours.
You can control the percentage, more or less, with the line:
iGoldStolen = ( iGold//25 )
However, to prevent this bonus from getting out of hand and to handle situations where the rival might have no gold there are also a few lines for minimum and maximum gold so adjust those accordingly if you change the percentages.
This really isn't meant to be used stand-alone but for modders who want to include new promotions outside the traditional XML bonuses.
All the python is labeled and searching for 'Marauder' should find the changes.
I recommend downloading and using Winmerge to combine python.
Edit: Added additional code that informs you how much gold you've looted.
Marauder Promotion (XML+Python) 2016-10-05
Marauder Promotion (XML+Python)