DeathBlow on a weapon is the same as the Death Strike skill. It gives a small chance to kill an opponent in one hit. If you normally kill particular monsters in two or sometimes three hits, it is kind of amusing to have DeathBlow kick in on the third hit. I had a sword at the end of the game with level 3 DeathBlow. The effect shakes the screen, so if it happened more frequently, I would have had to switch swords.
From RPG Vault,
Divine Divinity Skills XI. Links to other skill sets are at the bottom of the page.
Death Strike (Passive)
Level 1: 1 percent chance that the enemy is killed. If the strike is successful, the opponent must have a spiritual resistance of 10 to survive. Required Clvl: 24
Level 2: 2 percent chance that the enemy is killed. If the strike is successful, the opponent must have a spiritual resistance of 20 to survive. Required Clvl: 30
Level 3: 3 percent chance that the enemy is killed. If the strike is successful, the opponent must have a spiritual resistance of 30 to survive. Required Clvl: 36
Level 4: 4 percent chance that the enemy is killed. If the strike is successful, the opponent must have a spiritual resistance of 40 to survive. Required Clvl: 42
Level 5: 5 percent chance that the enemy is killed. If the strike is successful, the opponent must have a spiritual resistance of 50 to survive. Required Clvl: 48