Assassin's Creed Origins

Story-related, cannot be missed.

This is the only story related achievement in the game. As far as progressing through the DLC, refer to either a guide by IGN HERE or one by gamespressure HERE if you need help figuring out what to do next, but in general the DLC story quests are very simple and straightforward.

The only quest I'll highlight is the "A Pharaoh's Shadow" sub quest, where you need to find and defeat Rameses Shadow, since there is no objective marker for it. Essentially all you can do is hope to get lucky - you need one of the randomly-spawning Pharaoh's Shadows to be Rameses, and then you need to defeat it. I defeated 4 Pharaoh's Shadows before it was finally Rameses. I've looked at various guides and there doesn't seem to be a consistent way to get the spawn to be him. Some guides say go to Thebes, some say other things, but unfortunately it seems to be completely random. I recommend just fast traveling to different towns and changing between day and night until you fight Rameses. There doesn't seem to be a guaranteed way to move through this quest easily.

Tutankhamun is the final boss of the DLC, and defeating him unlocks this achievement. You should at least be level 50 or 51 before attempting this. Fighting him isn't really any different than fighting the rest of the Pharaoh's Shadows. Here is the tactic I used:

1. Immediately use your overpower attack (assuming you have the skill to start all fights with it, which you should by this point) and then immediately hit him 2-3 more times, then ROLL AWAY. Staying close for too long results in him doing an unblockable grab attack that takes off half of your health.
2. Now I recommend hanging back and shooting him with your bow. It won't do much damage at all, but it'll build up your overpower meter. Once it's full, use it and attack again like the previous step.
3. Repeat those steps until you're out of arrows. If that happens, then you'll have to continually dodge with and wait for a brief opening. Strike 1-3 times (don't get greedy!) and then get away before he can destroy you. Do this to build up your overpower meter and use it like before. Repeat until he's dead.

Once you defeat Tutankhamun, the achievement will unlock but you'll have to do a short bit of quest and dialogue to complete the quest and get your XP.