The Security Fleet[1] was a Forerunner naval unit.
When the Flood began actively attacking the Forerunner core systems, the Security Fleet, along with the Suppression Fleet and the Emergency Circumstance Fleet, were ordered by Forerunner Fleet Command to defend the core systems from infection by the Flood; however, they were most likely overrun by the the Flood and eventually perished.
Sources
| ||||||||||||||
The forerunners attacked the flood after there "key to survival" was destroyed by the flood.