Divine Divinity Walkthrough: Quests for Captain Mitox
After you help the Aleroth healers in "Divine Divinity," General Alix will ask you to deliver a note to Captain Mitox. This "Divine Divinity" walkthrough will help you complete the quests for Captain Mitox and get an invitation to Stormfist Castle.
Divine Divinity Walkthrough -- Orc Supply Train Quest
Take General Alix's note south until you reach the Army Camp, located in a village ravaged by Orc attacks. Captain Mitox will be in or around the main building. If you read the note before you deliver it, he will get upset; you will not, however, lose access to any of his quests.
Captain Mitox will first ask you to blow up the Orc supply train, which is helping to bolster their army. He will mark the supply train location on your map and give you some explosives. He'll also give you the human teleporter scroll, which you can use to activate the teleporter in the burned village to the south.
Waltzing straight over to the Orc supply train is a deadly proposition for a low-level character. Single Orcs aren't too tough, but in this area of Ferol you can easiy get surrounded. Thankfully, there's another way in. A bit north of the ravaged village, Egon will tell you about a secret mine shaft entrance in his house, which is at the southwest of the village.
Down in the mine shaft, you'll encounter a few solitary Orcs, including your first Giant Orc. Giant Orcs have far more hit points than regular Orcs, so they can do a lot of damage even if you're well-protected. Your best bet is a combination of Freeze and Poison Weapon.
The southeast exit of the mine shaft puts you in the middle of a large group of Orcs. The best way to deal with this is through hit-and-run tactics: kill an Orc or two, and drop a Scorpion Trap if you can, then duck back into the mine shaft to drink some potions.
Once you've cleared out enough Orcs, drop the explosives into the middle of the supply train. They'll explode in a cut scene while you proudly walk away.
Divine Divinity Walkthrough -- Orc Water Supply Quest
Return to Captain Mitox to receive about 15,000 XP and +3 Reputation. Impressed with your work so far, he'll assign another quest: poisoning the Orcs' water supply. Captain Mitox will mark the water supply on your map and give you a bottle of poison.
Once again, your safest bet is to travel through the mine shaft. This time, however, take the northeast exit. Fight your way to the water supply (or sneak, if you're a Survivor and it's nighttime), then drop the poison in the well. Return to Captain Mitox for another ~15,000 XP and +3 Reputation. He'll then send you back to Captain Alix for a reward.
In return for your bravery, Captain Alix will give you an invitation from Duke Janus to enter Stormfist Castle. Completing Captain Mitox's quests is the easiest way to get this invitation, which you need to continue the main plot. Since you'll get stuck in the castle for a while, however, you should level up a bit and gather some good equipment before you go in.