Fergason:
You came back. It wasn't difficult, was it? We don't need to arouse any suspicion. Now, go to meet Aku.
I already sent a message to Aku. He must be waiting for you.
To meet Aku, go to the South of the Seed of Hellfire through the Transportation Assistant.
After meeting Aku, kill about 200 monsters here in the Seed of Hellfire.
I know it's a little cumbersome, but you should make them believe you're working on something. On that note, I'll see you later.