system
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human
[Star Wars] How exactly did Darth Nihilus' hunger and powers work? How was Visas Marr the sole survivor of Katarr?

They were quite vague in SW: KotOR II about how he satisfied his hunger.
gpt
Darth Nihilus was a walking wound in the Force. Think along the lines of a laceration on a human body, except it's in the very fabric of the Force itself.

He was present on Malachor V when it was obliterated, and much like the Jedi Exile, his connection to the Force was violently severed due to the trauma of millions of lives being snuffed out at once.

A wound in the force is like a black hole of suffering and darkness, Nihilus was consumed by his hunger, trying to fill that "gap" in himself left by his severed connection with death and destruction. 

He needed to feed on the force to remain strong and drew in individuals who had a strong connection to do so.

He chose Visas Marr because she survived. The reason is irrelevant, if not her, somebody else. She felt when Katarr died what Nihilus himself did at the moment Malachor V was destroyed. She identified with him, and he took her as a kindred and a student.

TL;DR: Nihilus fed on the force itself and absorbed it from people around him like a walking black hole.