Capture the mines near the town if you haven't already. You can identify them as grey icons in the minimap. There should be a gold mine and an ore pit.
Then south from the town. Be careful though, you will soon run into an enemy hero. Enemy heroes can explore the map and become stronger every day just like your heroes. They may also attack if they see you.
Look out for circular like buildings - crystal caverns. Capture those as part of your quest. There should be two of those to the south, one of them guarded by infernal forces. By now you should have captured all the four mines to complete that objective in the quest.
Watch for Daeva, the enemy hero. Attack her when you find her, or there is a possibility that she will find you! This battle will be a bit harder then previous battles since you are fighting another hero, and also since that hero has more units then youre used to. Note that you cannot retreat from this battle so be prepared.
Be careful in this battle, enemy creatures take orders from a hero.
Like your own hero, enemy heroes can use abilities but can not be hit directly. You will still need to destroy all the enemy creatures in order to win.
It is recommended you focus on killing one enemy at a time rather then spreading your units against different enemies. This will be a tough battle so plan carefully. Cast your beneficial spells on the units and dont forget to attack every turn with the hero, preferably at the beginning of the turn.
After defeating Daeva, you will unlock some new quests: Journey through the Dark and Demon Blood.