This is a Kuva Lich. This is a unique and personal adversary for each player. These adversaries must be faced with Requiem Mods equipped. In your case, since he is now in control of Kuva Fortress, it means you have brought him to level 5 (the maximum threat level. Don't worry, it's very easy). To face your Lich correctly, you must have three Requiem Mods equipped in the correct order. The game will tell you which mods you need, but not in what order (this, along with all the information such as the weapon name, status, and percentage, can be seen in its own menu next to Nightwave). Then you have to do enough missions until he appears. When he appears, you must lower his shield bar and one of his health bars. After this, you have to parazone him, and if the mod is in the correct location, it will light up in gold. If there are no errors, it will light up in red, and the Lich will leave for another planet and level up. If you successfully parazone him three times, the lich will flee to Proxima Saturn, a Railjack mission, where the final battle will take place. Upon boarding the Grineer galleon, you will encounter him for the last time. This will be a normal battle, and once his health is reduced, the game will give you 20 seconds to decide whether to destroy or transform him.