Trigger Quest 4: Betrayal of Harrogath as you rescue Anya to the town and conversation with her in Quest 3, in which you are required to defeat Nihlathak.
Quest 4: Betrayal of Harrogath
A teleport(TP) appears in town after the dialogue between you and Anya, entering and reaching the entrance of the Temple where you need dispose a crowd of zombified warriors or run into the Temple instead. Monsters you meet here are what you've met before. Just walking along the edge of the map to arrive at the Halls of Pain, going on walking in the same way until you find the entrance to the Halls of Vaught. It is helpful to find a Waypoint in the Halls of Pain before entering it. Lighting the Waypoint, TP given by Anya disappears.
Arrive at the Halls of Vaught and walk in the four directions to increase experience as well as D2R items. Find a certain eye-shaped red wall tiles, and Nihlathak' eyes will appear at the end of room in front of the wall. He will summon minions and teleport, decelerating you with Arctic Blast. Concentrate your mind to fight with him. After you win, go to talk with Anya, and you will get a chance to name items as reward.
Quest 5: Rite of Passage
Getting rid of Nihlathak, you can see some hints about Quest 5 by pressing "Q" button. By Crystalline Passage Waypoint, you need find Glacial Trail first, through Frozen Tundra and reach the Ancient's Way by Frozen Tundra Waypoint. On the way to the Summit from the Ancient's Way, three formidable Barbarian Ancients possess different skills. Defeating them will gain a lot of experience and the entrance to the Worldstone Keep will appear.
Quest 6: Eve of Destruction
The Worldstone Keep eroded by Baal is the final battle place with large map and strong monsters. With the experience in Quest 5, you can defeat it. There are four floors in total, no certain way to the second floor on the first floor. Find the Waypoint on the second floor which is the departure place toward Baal. Turning left at the Waypoint, you'll find a way to the third floor where the map is extensive and irregular. So the last floor-the Throne of Destruction relies on your grope.
You can find the Baal at the upper right. Here I recommend you to eliminate all the monsters summoned by Baal with the help of mercenaries in its throne hall first for better gain as it summons 5 times of mini-Boss with high damage. among the 5 times, the damage in the second and the last time of mini-Boss are the highest. If you are less powerful, it will be kind of difficult for you. The best way is to lead them out of the hall and eliminate them respectively. Wipe out all the monsters, enter the red door behind Baal and challenge it (Baal will enter first).
It's relatively tricky to deal with mana rift halves, hoarfrost, incineration nova and mirror images of Baal. But it's not a problem when you have high damage. Damage of the mirror images is higher when its main body is in low HP(hit point), but blood drops much faster than the main body. When facing with Baal for the first time, most characters can choose to drink more potions so as to strive for more time to vanish Baal.
Actually you'll find that damage of such Boss as Diablo, Baal aren't very high. The utmost difficulty is to eliminate mini-boss. Quest 6 is completed as Baal is eliminated. Conversation with Tyrael, entering the red door, new quest will start. Passing Normal, Hard and Hell successfully will get a title from each.
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