This page of the guide to Deathloop has a walkthrough of the Visionary lead/mission Devouring of the Lambs. It explains how to infiltrate the masquerade ball quietly, where to find the DJ's machine and how to use it to unmask Aleksis, and how to get rid of the Visionary after finding him among the masquerade guests.
Unlocking the mission
You will be able to start the described lead after completing Colt's prologue - The Longest Day The mission will appear in your journal after looting a safe in an apartment in the Updaam district and finding a document with the personal information and whereabouts of 8 Visionaries (bosses and main antagonists). However, we advise you to proceed with this task after you complete the Ubiquity mission. This will allow you to use the Residuum to keep the powers, weapons, and other equipment acquired during the expeditions permanently.
To get on with the described lead, you have to visit Updaam in the evening.
Crash Aleksis's Party
Aleksis is staying at the castle, where there will be a wolf masquerade in the evening. The main path leading to the castle is heavily guarded, and choosing it is not a great idea. You risk losing life and, in addition, unnecessarily alerting those who are inside the castle.
We recommend choosing a path running along the walls of the castle. It is adjacent to the precipice, so it is worth being careful and use Shift when necessary. Finally, reach the side window from the picture above.
Search for clues
You'll find yourself in a small room. Get out of it onto the narrow window sill from picture 1. Walk a little along it and turn right twice. Take the stairs to the upper level of the castle.
Go straight ahead and enter the first door on the left. You'll get above the ball room shown in picture 2.
By default, Colt doesn't know what Aleksis looks like. If you do not want to risk eliminating wrong opponents, you will have to do additional steps allowing the character to identify the Visionary.
Start by getting to the shutters at the other end of the ballroom. It is best to use Shift to teleport first to hanging objects and then to the window.
On the floor, you can meet 1 or 2 guests, and you should quietly eliminate them. There is a device nearby that you can hack and distract enemies.
To find a clue, go to the right and examine a DJ machine from the picture above. You will have to complete a puzzle of finding Aleksis' favorite melody so that he would appear on the dance floor and get unmasked.
Enact the plan
Stay on this floor and go around the dance floor. Reach the stairs adjacent to the parquet and go to the upper floor.. Near the stairs, you will find a door that will lead to toilets.
Next to the toilet, you will find a second controller. You have to remember its combination, which is unfortunately random.
Go back to the first DJ machine and start pressing its buttons to copy the combination found in the toilet.
After you have done this correctly, wait a moment. Aleksis will appear downstairs, and when he speaks, you'll be able to confirm him as a mission objective.
The most obvious option involves attacking Aleksis from the upper balconies - but mind that you will alert the whole area that way, so you have to be ready to escape from the castle (preferably by the same route).
If you have a silent weapon, you can try to stealthy eliminate more people from the dance floor because they will have problems locating Colt.
The alternative idea is to go back to the previous room and set the buttons on the controller differently so that Alexis will stop dancing. Exploring the side of the room will allow you to find trapdoor release button. You can wait until Aleksis gets on it and starts his speech.
Additional hint - you can also get rid of Aleksis with the help of the trapdoor without solving the puzzle with the DJ machine. Once you reach the button, you can wait and watch the next person approaching the microphone. You'll recognize Aleksis by a man's voice and two orange weapons.