Astius Claenia's Hideout is a cave hidden in the Boethiah's Spine northeast of Llothanis.
As befits a hideout, the entrance is not easy to find. The best way of finding it is to head generally east into the hills from the Ruinous Crypt and keep your eyes open.
Inside it becomes clear what Astius spends his time doing: there is a bottle of greef, two of sujamma and two of mazte. Apart from a couple of items of iron armor and some food ingredients, there is little of interest. No indication is given as to what Astius is hiding from.