A guide on how to unlock the Freedom Cry DLC related achievements in AC Black Flag and access Avelin DLC

The first step would be to download the following two game from your steam library:

1. Assassin's Creed: Black Flag
2. Assassin's Creed: Freedom Cry

You have to do either of the following two:

1. If you already had been playing the game, you already have this and can skip to the next step.
2. If, like me, you downloaded this game to sync your Steam achievements with the Ubisoft Connect ones, and found that you had lost your save file, you have to play the first mission, say, till you reach Havanna with Bonnet.

Next, exit to the main menu. You'll see your save progress shows 1% (if you're in Category 2) or whatever your % is (if you're in category 1)

the file is located within:
C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\savegames\"Huge Cthulu mathematcial incantation"\437\

Cut and paste the 1.save and 2.save files to some other folder, say Downloads or Desktop.
While you're at it, open the Ubisoft Connect app> Profile> Settings> Uncheck the cloud save option

What I found was, that maybe if you load the game with a save file that could access the DLC from within Black Flag, that save file would still be able to access the DLC. You could find your own such save file. I used the following save file and it happenned to be one such save file: save[www.nexusmods.com]

Download the save file. Now this save file won't work right away if you put it within the same folder as your own save file's original location. For that you need to download the ACSaveTool from here: ACSaveTool[www.nexusmods.com]

Follow the following steps perfectly:

1. Put the near 100% save file from the link above in the folder C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\savegames\"Huge Cthullu mathematical incantation"\437. This folder should now be empty if as mentioned in the previous section, you have cut and pasted your own save files to a seperate location
2. Open the AC Save tool.
3. Click on the 'Open' button beside Save files and navigate to the files you just saved within the folder '437'.
4. Select the Game as Assassin's Creed IV Black Flag
5. Check the box in front of detect (decryption ID)
6. Click on the "..." button in front of Encryption ID field.
7. Navigate to your original saves which you had earlier saved in the Downloads or Desktop folder (as suggested in the previous section).
8. Click on the "Convert" button.
9. Close AC Save tool.
10. Open the "437" folder to check if there is a new folder created inside it named backups. If so, you have successfully converted the save files in the folder "437" to be registered under your ID. If no backup folder was created, you probably did something wrong in the above 9 steps. Try again.

Now boot up the AC Black Flag game and in the main menu, the save should say 95%. Ignore that, 'cause you're here for the Freedom Cry DLC accessible from the tile in the top right. This was earlier greyed out in your original save. Now click on it, and your have access to the Freedom Cry DLC from within Black Flag. Play it to unlock the achievements.

As a bonus, you also have access to the DLC named Avelin if you purchased the Season Pass (I guess)

Once you have played through the two DLCs and would like to access back your own save file of AC Black Flag (if you belonged to the category (1) from above), you could simply replace the 95% save file with your own original save file again and you'll be good to go