This path appears when you reach Varant. It doesn't matter if you followed the path of the rebels or the Orcs so far (maybe it was your own). You just have to know about the Adanos artifacts.
The path of Hashishin means close cooperation with Zuben. You need a high reputation to enter Ishtar so you have to visit a few towns in Varant and do quests for Hashishin. When you talk to Zuben (you have to pass his test earlier), you get some quests to do.
Your step is to learn Xardas's plans. You probably visited him so far so tell Zuben the story Xardas told you. Then the leader of the Hashishin orders you to fin Adanos artifacts and bring them to Ishtar.
I described earlier how to find artifacts and this time it doesn't matter if you kill Orcs or not. Get the artifacts and go back to Varant. Your next step is to put these artifacts on the Beliar's altar in Bakaresh (with the staff of the Eternal Wanderer received from Zuben). Go to Bakaresh.
You need a high reputation to enter the temple (you may have it already). Put the artifacts on the altar and pray. You get the Beliar's power and new orders.
Beliar's missions mean total extermination. You have to kill:
Xardas - go to his tower and kill him.
Rhobar - go to Vengard and kill the king.
Zuben - go to Ishtar and kill the leader if the Hashishin.
Orc chieftains - you have to kill all the main Orc leaders: Uruk from Cape Dun, Varek from Montera, Umbrak from Silden, Grok from Geldern, Vak from Trelis and Kan from Faring.
When you do that go back to Beliar.
This is the end of the Hashishin path.
This is a summary only of course. I ignored a lot of quest from the cities you visit, not mentioning locations from all over the world you may, but don't have to visit. These paths are only summary and it's up to you to choose other actions.