I've been testing different scenarios regarding rumors about how to get the addon.
After some tries and consulting with some friends that already got it, I came to the conclusion that in order to get the Addon 1 of the Ancient Aucar Outfit:
You need to stay 2 minutes at the boss room, while wearing the broken iks set.
Now, to clarify the Broken iks set consist of 4 items:
Broken Iks Headpiece
Broken Iks Cuirass
Broken Iks Faulds
Broken Iks Sandals
Weapons like the the Broken Iks Spear or the Broken Macuahuitl are not required to get the addon.
Keep in mind that, after the 2 minutes has passed while wearing the Iks set the items will be consumed and you will get in that moment the Addon 1 as well as the "Challenger of the iks" title (this one will appear once you relog, as every unlocked title).
Having say that, there's no need to kill Ahau to get the addon, you can spent the time at the boss room as you want and once the 2 minutes has passed you get the message and you can leave the room even without killing Ahau once single life.
I recorded how I get the addon so people can check it too.