As the name suggests, this is the killing strike you land on the enemy. An enemy will be open to deathblows once their HP bar is gone or as soon as you break their posture. Deflecting (parrying) enemy attacks can cause high posture damage to them while also protecting you from damage. Once a deathblow is available, you’ll see a red dot on the enemy. Press R1 to trigger the deathblow animation.
The easiest way to perform deathblows is through sneak attacks. There are different ways of performing stealth deathblows. The common method is by sneaking behind an enemy, close enough to see the red dot appear. You can also perform an aerial deathblow by jumping above an enemy and pressing R1 as the red dot appears, deathblow from a corner while hugging the wall, deathblow from below while hanging from the ledge, and so on.
Bosses will have deathblow counters, red dots on top of their HP bars. These represent how many deathblows you need to land on them to defeat them. By normal means, this includes slowly and steadily whittling down their HP gauge until you get the deathblow prompt.
However, it’s important to know that you can sneak up on some mini-bosses and perform a stealth deathblow on them. This instantly removes a deathblow counter on them, practically taking out a whole HP gauge. Try to perform this sneak attack whenever you can to make your life a lot easier. You can’t use this trick against main bosses.