When a game is created, players have the option to control how much luck will affect the game. They can pick an amount from 0% to 100%.
The default is 0%, as well as the ladders.
To understand the details of how the luck percentage affects combat, it is assumed you first understand the combat basics.
When an attack happens, WarLight calculates the "expected" number of armies that should get killed, assuming an average roll. The attack is also randomized as normal and these two numbers are interpolated based on the luck percentage.
If a player attacks with 100 armies, it is expected that it will kill 60 of the defending armies, but let's assume the random number generator determined that only 54 would get killed.
In a game with a luck percentage of 100%, WarLight will simply use the 54. However, in a game where the luck percentage had been reduced to 50%, WarLight would take the average of these numbers and would use 57 instead. If the luck percentage were 0%, WarLight would simply ignore the random and use the 60 every single time.
In the event that the calculations do not come out evenly, the rounding mode will be used to determine how the result is rounded.