“The blight is making it difficult for Ghost to get a fix on the coven of Witches. The blight must be cleansed.”
There are 4 Blights that are spread through the Steppes in Old Russia.
If you head over to the left and toward the nearest Blight. Be careful because just over the area you will encounter Consumed Vandal. This Vandal poses a fair threat because it has a good vantage point to fire down on the Taken Portal. Take it out and then destroy the Portal. If you are having a lot of trouble, change to a Sniper set up and counter-snipe the Taken Vandal.
With the first one destroyed then it is time for the next one. Head over to the right and toward the entrance of the nearby bunker, as though you were heading for Dock 13. Take out the Taken around the Taken Portals. Then focus on the portal. This makes things a lot easier for you.
There is another Taken Portal found by the water nearby the crashed ship. Clear out the Taken and then destroy the portal.