Even though the planet is bristling with the energy of the dark side, it is the Imperial military that has the largest visible presence on Dromund Kaas.
--The Old Republic Holonet
The powerful electrical storms were a physical manifestation of the dark side power that engulfed the entire planet—a power that had brought the Sith back here a millennium before, when their very survival had been in doubt.
--The Old Republic: Revan
The planet he was regarding with such intensity was either ocean or marsh, with very little dry land. It was a marked contrast with Korriban, which had been their last stop. But it was, to Luke’s mind, every bit as dangerous. Maybe more.[…]
"We need to watch one another closely. If Master Katarn could be swayed, then any of us could. Not just Vestara.”
[...]
Luke stood for a moment, his eyes and other senses searching the landscape. “Anyone sense anything?”
Ben extended himself in the Force, both opening himself to the vile sensation of the dark side and utilizing his senses—even smell, temporarily at least—to gather what information he could.
“Other than the obvious, which is a metric ton of dark-side energy, I can’t sense anything,” Jaina said.
[...]
“It’s... so strong,” Vestara breathed. Ben glanced over at her. Her brown eyes were wide, and her voice was a mixture of horror and attraction.
“Vestara,” Luke said, sharply but not angrily, and she blinked as if coming out of a daze. “Do you sense any signs of the Sith or Abeloth?”
She shook her head, looking more like her usual alert self. “No. That nexus pretty much drowns out anything else.”
[...]
Luke and Jaina were fighting back-to-back. The Sith attacking them had two advantages. One was the fact that they outnumbered the two Jedi. The second was that they were being reinforced by the emanations of the dark-side nexus within the temple. It surged forth like psychic sewage, clogging the Jedi’s reflexes as it fueled their enemies.
[...]
“I don’t think you did,” Luke replied. “They probably assumed we’d do exactly what we have been doing—investigating planets traditionally associated with Sith history. Khai probably chose this place because of the nexus. They could send a smaller team and still be stronger.”
--Fate of the Jedi: Ascension
Then there are places like the Emperor's Citadel and the Dark Temple that are focal points even within the nexus of Dromund Kaas itself, that have their own separate quotes:
Hundreds of years ago, the Sith Emperor ordered the construction of the Dark Temple as a burial place for his dead and defeated enemies, "to aid them in becoming one with the Force." Little is known of what rituals the Emperor performed there, but the Dark Temple has become a nexus of powerful dark side energy, and a place where ancient weapons and ancient secrets of the Sith lay sealed away in cavernous chambers.
Although the Dark Temple grounds have always been a dangerous place for the weak-willed (the expansion of the Kaas City power grid into tunnels beneath the temple drove a thousand slaves mad), the temple itself remained sealed until recently, when an expedition of power-hungry Sith Lords and their servants breached the gateway.
--The Old Republic Codex: The Dark Temple
The dark side power emanating from within the building was undeniable; he had felt the raw, crackling energy every day during his years as an acolyte.
--The Old Republic: Revan
"The dark side obscures my sight. We are walking into a time and place of shadows, and I cannot promise you we will ever come out."
--Revan, The Old Republic: Revan