The Derelicts Zealots started appearing in Warframe as part of Nightwave Series 2. Arlo is clearly up to something and all good Tenno must try to stop him. To do this, you will need a Zealot Derelict code. These will give you access to the Derelict and the mission it contains. Keep in mind that you won't be able to join the mission at all if you don't have a code, and you won't be able to get snapped up by someone who has one. All team members must have a code in order to start the mission.
Warframe - How to get Zealot Dropped Code
Getting Zealot Dropped Code is very easy; it's just a little ambiguous in the game. What you need to do is complete infestation invasion missions by infested epidemics. If you look at your navigation screen you will see a small counter above the Nightwave button that will tell you how many Infested Invasions are active and which planets they are on. This will only appear if there are active Derelicts on the star chart.
Just go to the planet, then look for the invasion icon on the nodes. You can also click on the Invasions tab in the upper right corner of the screen and select the mission from there.
Like all invasions, you have to complete the mission three times to secure your Battle Balance, which is an abandoned Zealot code. Once you have the code, you can access the mission, complete it, and get your rewards. The Abandoned are on a new node, orbiting the planet where the invasions are taking place, as shown in the image above.