Assassin's Creed Origins

This is a cumulative achievement, and thankfully rebels are very common. They are typically locked up in wooden cells in various encampments and fortresses. This was actually one of the first achievements I unlocked in the DLC because of how many rebels you can find locked up. Here's where to look:

• During the "Where the Slaves Die" main story quest (the first one), you'll go to a huge quarry with enemies. There are a ton of locked up rebels on one side of the quarry.
• During the "Wall of the Ruler" main story quest, you'll go to the Wall-of-the-Ruler citadel. You'll also have to complete this location for an achievement. Inside are always a bunch of rebels locked up.

I actually unlocked the achievement after freeing rebels in only those two locations. If you still need more, any fort (red icons on the map that are restricted areas) is a good place to look. Additionally, both enemies and rebels will respawn in the same camps. So you can use the Dawn and Dusk skill to fast forward time until the rebels respawn in camps. Just be aware you might have to skip forward a few days until the rebels respawn.

Once you've freed a cumulative total of twenty rebels from their cells, the achievement will unlock.