There are 3 hidden achievements and no one knows about them. I ask the internet, searched on the DoC Wiki and in the comments below. Even ChatGPT can't find something. They are truly hidden. I already got 2 of the hidden achievements, (mask on and off) but I cannot find a hint for the last one.
From what i hear last one is about corruption, also related to the mask if I am not mistaken, taking the companions and have em corrupted, then use the mask as when you used it for purity run but instead of the option of them being free of corruption its the one about them giving in temptation/corruption. I am going to try it out one of these days see if that is how one gets it