Caesar's Foe is the name given in the GECK editor to the quest which controls the appearance of Legionary assassins. Should the Courier's negative reputation with Caesar's Legion reach a certain point, Legionary assassins will be dispatched to kill them. As of patch 188.8.131.525, this quest has been dismantled and replaced with "Faction Hit-Squad Master Quest", with the same in-game function.
Legionary assassins are dispatched within hours of the Courier first attaining a sufficiently negative status with the Legion. Thereafter, assassins are dispatched approximately every three days, depending on whether or not the Courier is located in an area where they can be attacked.
A good area to get the drop on them is to move from Field's Shack straight past the chem junkies' campfire nearby. The assassins will spawn at the base of the nearby hill. You can ambush the Decanus running towards you, as they spawn just after you pass the two junkies. By quickly dispatching the Decanus the other assassins will usually stop running and end hostilities (provided you keep some distance) allowing you to relocate and easily pick them off with a decent player level and/or silenced weapon.
pcps3xbox360 Previously this quest would only cause Legionary assassins to appear once. Due to scripting errors any further appearances of Legionary assassins would not be triggered, regardless of the Courier's location or reputation with the Legion. This issue has been fixed as of patch 184.108.40.2065.
pc Given Boone is not a companion, wearing Legion Armor before the assassins are able to see you will result in the Decanus engaging in dialogue with the courier saying the the courier is marked for death. As soon as the dialogue ends, the hit squad will stay still, allowing the player to kill them one at a time with melee sneak attacks, given the player can kill any one assassin with one sneak hit.