After finishing the conversation with Ostegoth, proceed north again. Your current mission's destination is the [Black Stone]. Locate the main gate of the fortress, pictured above, and go inside.
Start exploring the Black Stone by going through the east door. Go upstairs to reach a small area where you'll meet Lilith. Notice that after the conversation the game had upgraded the Voidwalker ability to Phasewalker (the ability to travel through time using special types of portals). The current quest will end, so you can count on acquiring 2000 XP and 9000 gold. Another quest, [The Lord of the Black Stone], will begin automatically.
[TIP: The Phasewalker ability is used to activate single green portals (time gates) which let you travel between the present and the past. After using a portal you will end up in the same spot, but given an extensive time difference the same locations will look completely different.]
[TIP: Unlocking Phasewalker doesn't mean you can't use Voidwalker anymore. Blue portals are still available and to activate them just use Phasewalker from the ability list.]