Raider Bases Missions are unlocked at player level 10 and range from 3 to 5 stages depending on rarity, and each stage will cost 3 energy. On the map they will have an icon of three Raiders.
From a Common Raider Base you might find the Epic Regulator AR, from Rare Raider Bases you might find the Epic Inhibitor SMG, and from Epic Raider Bases you might find the amazing Legendary Guardian Sniper Rifle!
Raider Bases are dynamic spawns instead of always spawning in fixed points. This means the only way to have them re-spawn again in the same place after completing them is by using a terminus map. In that way they are similar to rare and epic infestations.