Online:The Serpent's Fang

Quick Walkthrough
# Meet Little Leaf in Ilthag's Undertower.
# Kill Ilthag Ironblood.
# Talk to Little Leaf.
# Meet Little Leaf at the Valley of Scars.
# Kill the armorer and the runescriber.
# Enter Exarch's Stronghold.
# Kill Exarch Braadoth.
# Talk to Little Leaf outside the stronghold.
 

Detailed Walkthrough
Little Leaf informs you that Regent Cassipia sent her here to convince the Iron Orcs to disband their army of trolls. However, they took her gold and tossed her out. Following your arrival, she has come up with a new plan, and asks you to meet her in Ilthag's Undertower. She's going to forcefully convince the Iron Orcs to stop building their troll army, and she wants your help.

The Trainer
Once inside the tower, you can ask her about Ilthag Ironblood. He is an Iron Orc battle trainer. She tried to bribe him to stop training the trolls, but since that didn't work she's going to have to "beat him repeatedly" instead. 

Climb down the ladder and make your way north. The first area in the Undertower is full of Iron Orcs. Defeat them and enter the troll arena-like area. This place has Scaled Court enemies, Uzka Trollfeeder, and a large troll named Killraken. Once they are defeated, a group of Spellfiends and a Blackcaster Storm Mage appear along with a huge storm atronach named Stormbringer. Go up the stairs and face more Iron Orcs. Down the hall, and up more stairs, are more Orcs and a Skyshard. Push past the final group of Orcs, then turn right. Ilthag Ironblood, Vosh, and Rahk, plus a few more Scaled Court enemies await you in the small room. 

Defeat them, then talk to Little Leaf.
:'''I'm not sure killing the trainer is going to put an end to the troll army.
:"Good point. What was I thinking? We're going to have to do a lot more to disband the troll army. I guess I need to talk to Runescriber Kulth, Armorer Uthik, and maybe even Exarch Braadoth."
:You might be on a first-name basis with the Orc leaders, but diplomacy isn't going to work.
:"You're right. I know you're right. That doesn't make it any easier. Still, I have a mission, and if violence is the only way to complete it, then violence it is." 
:So where do we have to go?
:"The Valley of Scars. And, yes, the place is as delightful as it sounds. It's north of here. You can't miss it. I'll meet you there."''

You can ask her about the three Iron Orcs you need to "convince" to stop. This reveals where they are and their role. The runescriber adds nirncrux dust to his ink to boost the magic and is likely at the west side of the valley. The armorer adds nirncrux dust to make her armor stronger and is probably at her forge on the east side of the valley. Exarch Braadoth is the Iron Orc chief and also a ranking member of the Scaled Court. His full title is the Ophidian Exarch of Undulating Destruction. Little Leaf is sure he is in the valley somewhere.

The Valley of Scars
Head north to the valley. Once you enter the valley, the quest should update, and you have to kill Runescriber Kulth and Armorer Uthik. The runescriber is well-guarded by Iron Orcs and trolls. There is a named troll, Oruzkad, in the longhouse with the runescriber. Watch out for the fiery rain of arrows and the large rocks the troll throws at you, which knocks you down and takes almost half your health. The armorer is accompanied by a troll and a welwa, and also has a few more trolls around her.

Now, you need to enter Exarch's Stronghold and defeat Exarch Braadoth. The Throne Room is at the back of the complex, and you will need to fight your way across a bridge and through a number of chambers of Iron Orcs, welwas, and trolls to reach it.

Once he is defeated, Little Leaf runs off to send a message to Cassipia. Talk to her outside Exarch's Stronghold to complete the quest.
:''"So that's how he did it! He was imbuing the trolls with pure nirncrux! Fusing the armor and the runes into a powerful whole! I wonder what would happen if one of the mortal races was subjected to the process?"
:That won't happen. Braadoth is dead and the process has been disrupted.
:"Right. Of course. Cassi asked me to bring her some of the altered nirncrux to her. So she can study it. I'll stay here and collect what I need. But thanks. I couldn't have completed my mission without you!"''
  

Quest Stages