The Andasreth propylon index is functional with the Hlormaren and Berandas propylon chambers.
The index is located in the dome of the Chimer fortress Hlormaren, on the Bitter Coast north of Hla Oad. The dome is a small and separate chamber on the rooftop of the stronghold, so it can be accessed by levitation without a fight; otherwise, the only route would be the corridors of the Hlormaren keep, which involves either some excellent stealth, or some closequarters fighting with the resident outlaws.
Once on the rooftop, there are two more outlaws - Brilnosu Llarys and Nevon Verilnith. Unlike those inside the fortress, however, they will not attack without provocation. The way to the dome is open.
Inside the dome, the index is located on a shelf to the left of the entrance. Two Khajiit slaves are present - [Insert name here.] and [Insert name here.] - but taking anything out of the room spawns nothing more than a rather half-hearted verbal protest, so there's no worry about grabbing the index in their full view. The only challenge that can befall an adventurer at this stage would be the sheer lack of light; the dome is very dimly lit, and without a nighteye enchantment, the index can become invisible against the dark wood of the shelf. To aid the straining propylonrider as much as possible: the index is located on the second level of the shelf, near a bowl and some bottles.
Once the index is in an adventurer's position, it can be taken to Hlormaren's propylon chamber and used immediately, because Hlormaren links to Andasreth on the propylon network. This gives the Andasreth index the distinct honour of being the only index you can test immediately after its found. The whisk from Hlormaren's trashed chamber to Andasreth's empty one doesn't afford that much of a thrill, however.
The ability to teleport to Andasreth from Hlormaren or Berandas isn't all that useful by itself, because Andasreth is located in a very remote location, and while none of the old Chimer strongholds are particularly cosmopolitan, there aren't even any other interesting ruins near this one. The Andasreth index is still, necessary, though, to complete the propylon transportation network, and travel from Hlormaren to Berandas - near Hla Oad and Gnisis, respectively - can come in handy.