The detection rule "EC2 Instance Started With Previously Unseen Instance Type" aims to identify unusual activities by tracking the creation of Amazon EC2 instances using instance types that have not been previously logged or recognized. Utilizing AWS CloudTrail logs, this rule captures the event where instances are launched and cross-references them against a lookup table of known instance types. The rule flags any instance type that has not been seen in the last 70 minutes, indicating potential anomalous behavior. The rule is marked as deprecated and is suggested to be transitioned to using the most recent Change Data Model for better performance and compatibility. Implementing this rule requires the installation of the appropriate Splunk applications for AWS and running preliminary searches to populate historical instance type data.