Sadism is still a thing wich isn't quite clear. How to gain sadism, what does sadism. And there is some info wich is outdated cause sadism has changed since the begining of the beta.
Sadism is gained very early as a passive ability (around lvl 10 or probably earlyer). Sadism procs when someone in your group gets hit. Sadism is a self buff only. (it was a group buff before). Sadism can be stacked up to 10 times. The sadism buff wears out if he isn't refreshed after 10sec. If your sadism counter reaches 10 you begin to pulse aoe unholy damage every 3sec.
Sadism gives you every stack +1% damage +1% lifetap +1% magical damage +1% magical lifetap
There are many feats wich are connected to sadism:
Depravity tree:
Vile Soul tier 5
2 Point Feat
Effect: You will start to pulse aoe damage when your sadism counter reaches 8 / 5 rather then 10.
Masochism: left depravity tree tier 6 feat
5 Point feat
Effect: There is a chance that you add a sadism counter when you get hit
Depravity: left depravity tree tier 7 feat
5 Point feat
Effect: Your sadism counter will be raised by 1 everytime covenant of vengeance procs
Desecration tree:
Ritualistic Bloodshed tier 5
2 Point Feat
Effect: Gives the Dark Templar a clickable ability to increase the sadism counter by one. However it does a bit of mana damage to the dark templar. The cooldown of the ability let you stack sadism to 10 and keep it up as long as you have mana left.
How to get to 10 sadism stacks as fast as possible and keep it up to get the most out of it:
If you have specced all the depravity feats mentioned above, you can stack sadism even if you aren't in a group. But you will only stack sadism if you fight (get hit or proc covenant of vengeance). And you can only keep it up by fighting. Contra: If you make a pause wich is longer than 10sec, sadism will wear out. And you have to stack it up again from 0.
If you spec into ritualistic bloodshed you can stack sadism by clicking on the ability. You can stack sadism before a fight to 10 and if you click it every time before the timer wears out you can keep it up forever. Ritualistic bloodshed does 15% mana damage from the remaining mana, wich means you can use it even if you have only 2 mana left. *changed: it now uses 15% of your total manapool. Spec into mana leech (and all the other mana related feats) to top up your mana* Contra: if you forget to press the ability and the timer wears out you have to stack again from 0. And its easy to forget to press the button, especially if you are engaged in a pvp fight.
Final note: you can see, that sadism stacked to 10 is a godly buff wich helps in every situation cause its passive and counts towards every hit and proc. My own experience shows that ritualistic bloodshed is superior to the depravity feats in terms of stacking sadism. In a group you will have stacked sadism quite fast, and ritualistic bloodshed helps to keep it up during pulls and pauses.