Saturas sends you up to the mage 
Xardas, but a demon bars the way into his castle and only opens it once you bring three Golem Hearts. That means tracking down and killing the 
Fire Golem, the 
Stone Golem, and the 
Ice Golem out in the harsh country near the tower, then handing the hearts to the demon and going up to speak with Xardas.
This is a rough corner of the map with hard fights, so come prepared and save often.
- Plenty of healing potions.
- Food or backup healing.
- Strong armor.
- A weapon with good blunt damage.
- Ice magic if you use it, which works well on the Fire Golem.
The surrounding area is thick with enemies, from orcs and Orc Hounds to packs of wolves up in the snow, so do not expect a quiet walk between fights.
Make for the dry, sandy mountains near Xardas's region, up in the high part of the map by the great tower and the lava. All three golems live out here, and the demon will not budge until you have a heart from each.
From the sandy ground, drop down the rocks and loop back onto the path. Bear right, then curve left to the flags. At the end, climb the wall if you have learned how, or scramble up the rocks by hand. Up top you should see Xardas's tower off to the left and lava to the right. Hug the mountain on the right and climb the stones toward the lava, and you reach the Fire Golem.
Against any of the golems, let it start a swing, dodge in or around it, land a couple of hits, then back off and reset. Ice magic chews through the Fire Golem if you would rather cast.
Beat it, loot the body, and take the first Golem Heart.
Head back down the stone steps to the sandy area by the tower. From there, go up the nearby hill, clearing or dodging whatever gets in the way, until you reach the Stone Golem. Use the same blunt-weapon, dodge-and-strike approach, then loot it for the second Golem Heart.
From the Stone Golem, go through the archway at the back and across the water. Where the path forks, take the right branch, follow the wall around, and at the next fork take the right again. Keep hugging the wall and jumping up the rocks until you reach a cave.
Inside, grab the plants for 
healing herbs if you want them, then climb and circle upward through the cave until you come out in the snow.
The snowy stretch brings white wolves and snow wolves, and they can be nasty in numbers. There is often a bad pack waiting in the big open section.
If you would rather not fight the pack, climb around the rocks on the side of the mountain to sneak past. The route is fiddly and a little glitchy, so move slowly and watch the edges, since a fall here hurts.
Once you reach the snowy cave, head in and you find the Ice Golem. Same plan as the others: dodge into its attacks, get your hits, heal when you need to. Kill it, loot it, and take the third Golem Heart. That is all three.
Leave the cave and head back down the mountain toward Xardas's castle, using the rock route again to skirt the wolves if you avoided them before. Drop back through the cave with the plants and exit to the right.
There is a quicker way down from here: hug the wall and carefully shimmy down the slope to reach the lower door instead of going the long way around. Do not drop too far at once, or the fall takes a big chunk of health.
Go in through the lower door and speak to the demon. Hand over the three Golem Hearts and it lets you pass. Head up, find Xardas, and talk everything through with him.
Once you are done with Xardas, go back to Saturas and report in to finish the quest.