After completing the Infernal Hero quest, make sure to go back to town to recruit reinforcements, as you probably suffered lots of casualities when fighting the enemy hero.

Now let's do the following quest:

Journey through the Dark
Now that the workers are back in the mines, they cleared the path leading to the underground tunnels. This shortcut will prove handy in reaching the Mausoleum of the Griffin family, on the other side of the mountains.

Active Objective - reach the other side of the tunnels

Rewards: 500 Experience

While doing this quest, we will also do the following secondary quest:

The Horn
(Secondary Quest)

Humans and Orcs have trouble fighting together, and morale is low. To improve things, Slava should seek the old artifact known as the War Horn of Legion. The sound of this Dwarven Olifant will life the spirits of any warrior

Objectives:  Find the Horn of Legion

Rewards: War Horn of Legion

Head south from town, until you reach this green misty building. This is the entrance to the underground caverns.

Enter the caverns.

homm6 underground caverns

In the Underground, first head south. You will run into some ghouls who are guarding the War Horn of Legion. Kill them to complete the horn of legion quest.

Then start making your way through the caverns toward the exit.  Head North, West, then South. Along the way  you will run into many ghoul, vampire and undead armies protecting various artifacts and resources. Kill them if you wish to gain those items.

Once you reach the Subterranean gate, double click on it to exit the underground caverns.