From what I have heard, I googled Aoc Dt feats, and it explained sadism as having a chance to stack every time you or somebody in your party takes a hit. It will show up where all your normal buffs appear (by your food)It is a red and black icon if I remember right, kind of like the Bear shaman blood flow.
I believe you can feat to make it easier to stack using mana, or stack when you DO damage instead of receive it.
Sadistic Soul has an Unholy damage proc on proximity whenever you have it running. Much like any other two soldiers Vengeance and Furious Inspiration stack, you get stacks whenever another member of your group gets hit by default. DT's in particular have two feats that allow you to bypass this mechanic, with Ritualistic Bloodshed and Masochism.
Of the two, it's widely accepted that only Ritualistic Bloodshed is worth feating (if you have to).