The TOR anonymity network is the perfect place to hide command-and-control, exfiltration, or ransomware payment through bitcoin. You would like to use firewall data to find TOR traffic on your network.
How to use Splunk software for this use case
For more great content from the Splunk Education and Training team, check out Splunk How-To on YouTube or sign up for a course. In addition, these Splunk resources might help you understand and implement this search: