This spyware is not available Dark Web it can be purchased legitimately. The Guardian calls it:
“…perhaps the most powerful piece of spyware ever developed – certainly by a private company.”
Both The Conversation and The Guardian have articles looking at how the powerful Pegasus surveillance software has been used and misused. The Guardian article has a detailed explanation how the software can infiltrate a device.
What is Pegasus spyware and how does it hack phones? | Surveillance | The Guardian