Whenever a living human land-based unit is defeated by another land-based unit, there is a chance that the unit about to be defeated switches allegiance and joins the victors. They keep all promotions, exp, and levels, lose 1/2 hp, and can't be moved until next turn.
I take no credit for this mod. I only updated it to Beyond the Sword out of my own desire to use the mod again... Amra
Extract this file to your Beyond the Sword "Mods" folder within the Beyond the Sword game directory.
For example, C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Beyond the Sword\MODS.
It should create a folder called "Unit Allegiance Mod" that will include all of the modpack files.
In game, click on "Advanced" and then "Load a Mod" to play.
If you want this mod to load automatically every time you play Civ4 Beyond the Sword then edit
the "CivilizationIV.ini" file in your "\My Documents\My Games\Beyond the Sword" directory.
Search for "Mod = 0" and change it to "Mod = Mods\Unit Allegiance Mod".
Spoiler Version Info and Credits :
[BtS] Unit Allegiance Mod v0.5 2016-10-05
[BtS] Unit Allegiance Mod v0.5