From Ex illis wiki
A vast number of events completely unforeseen by the strategist’s rigorous planning can happen during the chaos of a melee. A hit can touch a vital organ while a clumsy movement can cause a sprain. These small events are tagged under “Luck” and “Bad Luck” and create opportunities or threats forcing the players to adapt their strategy.
The Luck system operates during the Roll to hit. It adds 5% Luck and Bad Luck to most rolls. With a Luck, the Final damage is increased by 150% while a Bad Luck causes the warrior to inflict himself/herself 10% damage to his maximum HP. A few techniques influence the Luck probabilities as well as their effects. For example, basic actions like Fight, Ranged Attack and Charge are not affected by Bad Lucks (although they can still roll a Luck). Some players will find the tools to build interesting strategies according to their own tolerance to risk.
Tip: It is essential for a good player to learn to recognise the techniques which kill a specific enemy in a single hit in order to establish predator/prey relations on the tactical level. It can be wise to notice how Luck can also make an attack particularly deadly in one hit while it would normally have taken two.