The 90%-rule

Reducing the impact of luck.

Warbarons restricts the impact of luck on battles through its "90%-rule". Each battle is simulated a thousand times, and the number of occurrences of each outcome is noted. The battle is then performed one more time, to determine the outcome that counts. If the dice rolls in such extreme favour of one side this time that it would go beyond the 90% rules fringe outcomes then the battle dice will reroll until the battle passes the 90% rule. This is displayed with a dice symbol on the dead units showing that it was auto killed to stay within the extremes. To fully understand what the mechanism does here is a example of a battle.

Player Blue attacks with 8 light infantry against player Red who also have 8 light infantry. The outcome from this without any rule could be that blue win with 8 light infantry intact. With the 90% rule the following will happen, the possible outcomes from running this battle 1000 times will get results that looks something like this (not actual results):

Blue survivors								Red survivors
8	7	6	5	4	3	2	1	1	2	3	4	5	6	7	8
0.5%	2%	3.5%	5%	7%	9%	12%	14%	14%	12%	9%	7%	5%	3.5%	2%	0.5%

Looking at the table we can tell that the most extreme outcomes on the left and right side are that any of the players win with 8 units alive. It's only 0.5% chance that it will happen. With the 90% rule we will remove the outcomes until we reach 10% or more on each side.
0.5% 2% + 3.5% + 5% = 11% so 8,7,6,5 survivors is too good of a result but 4 survivors is okay.
The "fringe" meaning the extreme outcomes, where an outcome is (usually) more extreme the more survivors the winning side has. The effect is thus the elimination of extreme results, such as winning without losses against strong opponents, or losing many units against very weak opponents. The 90%-rule does not apply to ruin exploration.

Victory is secured!

A consequence of the 90%-rule is, that if the battle dialog of the game displays a win chance of over 90%, then the attacker will definitely win with at least one unit surviving. Similarly, the attacker will definitely lose all units if the game comes up with a win percentage below 10%. Note that Warbarons itself never displays chances that already account for the 90%-rule. So a displayed percentage above 90% actually means 100%.

Going to extremes.

Since the application of the 90%-rule depends on random-based simulation, any outcome still remains possible. A single weak Scout may kill 8 strong Dragons, but the 90%-rule makes the chance for this to happen very slim indeed: the Scout must win at least 100 of the 1000 battle simulations, and then he must also win the last battle that counts. This is extremely unlikely, but not completely impossible.
The 90%-rule may sometimes have weird consequences, such as smaller stacks having better individual survival chances: e.g. a single spider has a slightly better individual survival chance attacking an elf in a city with no wall, than a spider and a crow attacking together in this order. However, the overall winning chance does improve by adding the crow. The differences are quite small though.
Note that the winning chance shown by the game likely differs if you repeat a battle with identical units, due to the random chances within the simulations that were used to determine the win chance within the game. In fact, the game may produce any winning percentage, as the scout-vs-dragon example above shows.

The outcome percentage explained.

Note that the game already knows how the battle will end when the battle dialog has finished loading - it just has not shown you the outcome yet. The dialog also provides a spoiler through the outcome percentage display: this is the probability of the survivor pattern that this particular battle will have, with respect to the simulation. So a value of 40% means that around 400 of the 1000 simulations ended with the same survivors.
It is furthermore also possible that this outcome percentage may well be below 10%, despite the 90%-rule, since it might be a rare outcome in the middle of all possible outcomes. The 90%-rule only cuts the fringe outcomes. For an example, consider a stack having a very weak unit wedged in between strong units. If that weak unit reaches the front line and survives, then that outcome would be quite rare, even though it would be expected that the overall stack wins in any case.

The 90% rule is a variable between 79% - 90%.

The 90% rule is actually not locked at 90% but instead adjusted depending on the size of the battle. In a bigger battle luck is reduced even further to not let exceptional luck in a single battle decide the outcome of a otherwise equal game. Luck will still play a role but with reduced luck a player can predict the possible outcomes thus making it possible to avoid losing because of bad luck.The 90% rule is adjusted by the total battle value of all units including terrain bonuses but no other bonuses included according to the following table.

Total battle	percentage is use
Total battle < 65	90%
Total battle 65 - 100	89%
Total battle 100 - 145	88%
Total battle 145 - 190	87%
Total battle 190 - 235	86%
Total battle 235 - 280	85%
Total battle 280 - 325	84%
Total battle 325 - 370	83%
Total battle 370 - 415	82%
Total battle 415 - 460	81%
Total battle 460 - 505	80%
Total battle > 505	79%