The Aspect of Cruel Sustenance is a defensive-type Legendary Aspect in Diablo 4 that is specific only to the Rouge class.
This legendary aspect gives you a major portion of your health back, every time you deal damage to an enemy using your explosions from the Victimize Key Passive.
Since the explosion from Vicitimize key passive damages more than a single target, the chances of you getting back your health greatly increase. This legendary aspect can become a crucial item in your arsenal that can make or break a fight.
Make sure you are focusing on skills in the Rogue Skill Tree that favor the Victimize key passive such as the Shadow Crash ability which adds shadow damage to the explosions.
Aspect of Cruel Sustenance location in Diablo 4
Most of the legendary aspects have to be unlocked by completing dungeons in Diablo 4. However, there are some that have to be extracted from looted gear on which they are imprinted.
The Aspect of Cruel Sustenance falls into the latter category, meaning that it is an extracted aspect that is not part of your Codex of Power in Diablo 4.
Once you have the item in your hands, head over to an Occultist and extract the legendary aspect to be used on any other item of your choice.
You can then either use the item as is or you can take it to the Occulist where you can get the Legendary Aspect extracted, which will destroy the item, but now you can use the Aspect of Cruel Sustenance on any item of your choice in D4.
Your best chance at getting a Legendary piece of gear is by taking part in world events, taking down world bosses, and completing Dungeons, and Strongholds. All of these activities greatly increase your chances of Aspect of Cruel Sustenance.
Aspect of Cruel Sustenance builds in Diablo 4
The Aspect of Cruel Sustenance favors the Victimize key passive so every build that is centered around this ability is well suited for this Legendary Aspect. A build such as the Penetrating Shot Rogue Build.
The build focuses on dealing damage to multiple enemies from a distance which means that you will have a higher chance of causing an explosion using the Victimze Passive which in turn increases the odds of you getting your health back using the Aspect of Cruel Sustenance.
Some other examples of builds suited for the legendary aspect are Shadow Barrage Rogue Build and Trapper Rogue Build.