I know for a fact that you can't even get to where this HIDDEN entrance is till AT LEAST Sequence 6
If you try for it then, you may have to drop then ledge grab to get to the entrance.....if you wait till Sequence 7, you can fix all the aquaducts and just drop into the water safely......
Now for directions........., Head to any Tunnel Entrance and look for the "Terme Di Diocleziano" destination, fast travel there.......once you arrive, look to the north and you will see an archway, go through it and up the stairs....ahead of you will be a bridge with an archway behind it, don't go there, just look to your left and down, left of the bridge, you will see an opening to your left with a chest.......and there is the entrance to this Lair
If you want a video, please ask, this lair was way too straight forward to warrant me adding one.............to get 100% sync here,.....don't lose more then 10 health squares......one of the easiest syncs thus far