Due to the incident with Tirek, the magic-eating centaur who tried to take over Equestria not too long ago, Celestia has decided it'd be best if Cerberus's responsibilities were lightened a bit. She reasoned that Cerberus, while being an immortal, three-headed guardian of the prison of the most dangerous creatures and beings on the planet, is still a dog, and so should get to rest and relax once in a while.
Therefore, she decided that, in order to lighten Cerberus's load, she will send six ponies to guard the outter side of the gates of Tartarus. These ponies will be highly skilled in combat. There will be six more ponies to cover medical and delivery needs, respectively, and a Sergeant to make sure the guardsponies do their jobs.
This assignment is, essentially, a test-run see if adding more guards to Tartarus's gates is a.) necessary, and b.) safe enough (aka: not suicidal).
There are five spots open, for the guards. More spots can be available if someone wants to take the role of a nurse.
I will handle the Sergeant and the delivery ponies, when they're actually here.