Warcraft 3 Frozen Throne Walkthrough for Undead Scourge:
Chapter 4: The Return to Northrend
Main Quest: The Landing
Since there is nothing to be found to the northwest, take all your land forces directly west. Kill the trolls, then the polar bears to get some items. If you destroy the igloos you'll find 250 gold coins, which will surely come in handy. Give Anub'arak any rings of protection you find. He can come up with quite a lot of army over the course of the campaign, eventually becoming the most durable hero you've had thus far. Make your way up the slope just after the polar bears. While the Tuskarrs here will yield nothing, there is a giant penguin behind their camp which drops a ring of superiority. Give it to Anub'arak just because he has so little at the moment. Return your army to the shipyard and the waiting transports.
Buy an additional two battleships at the shipyard. Put all your navy into a single group and sail north. The 'Island Stronghold' you're to destroy isn't much. The most important thing is to hit the defense structures with your battleships. Have your transports close by so that you can make the landing as soon as it's safe. Your battleships could take damage from flying units if you're not careful, but as soon as you land the crypt fiends can make it much easier. Set your necromancers to auto-cast animate dead now, so that you can have extra forces to speed things along. Wipe out the entire base and your landfall will be complete.
You will start with plenty of gold and not so much wood. Build a crypt with an acolyte immediately, and make about four ghouls to harvest for you near a graveyard which should be placed on the eastern side of the base. Use Arthas and Anub'arak to break open the barrels littered around the camp. You'll gain plenty of extra wood doing this. Make some ziggurats to defend against enemy attacks, which will primarly come from the west and southwest. Space the ziggurats to the southwest out some so that they can better combat the cloud ability of dragonhawk riders. Don't upgrade your ziggurats to spirit towers just yet, instead aim for a total of ten crypt fiends to accompany Arthas and Anub'arak. Work on creature upgrades at your graveyard.
Optional Quest: Dragon Hoard
When you have your crypt fiends ready to join Arthas, upgrade all your ziggurats to spirit towers. Make a couple extra crypt fiends to guard your base while you're away, then head to the north with Arthas' army of crypt fiends. After the second battle with the blue dragons, you'll find a Sobi Mask, which is great for Arthas. Continue along the path until you reach Sapphiron himself. When you do, focus fire on one of the blue dragons first to bring it down. Note that Sapphiron appears to use taunt, so you will occasionally need to retarget the dragon you're focusing on. Use death coil as often as possible on units that are dying. I only had about five crypt fiends left when this was over, but the reward was well worth it. You'll gain an undead version of Sapphiron, which is like two bone dragons combined. You'll also get access to his dragon horde, which yields several tomes as well as some wood. Sapphiron himself drops the horn of the clouds, which can be used to stop enemy towers from attacking.
Main Quest: Breakthrough
The rest of this chapter is really easy with Sapphiron. Take Arthas with three abominations, Anub'arak, Sapphiron and six crypt fiends and board a boat to the west of your base. You can buy an additional three battlecruisers here. Use your Ships and Sapphiron first to soften the Naga base to the northwest. Build extra units to defend your base. When your battleships are ready at the northwest, destroy everything that is on the shore, and have Sapphiron attack any air units that come along. They'll die really fast. When the way is clear, Send Arthas' army in transports to land on the Naga beach. Now it's up to the land forces with Sapphiron. Crush the Naga, using death coil often on Sapphiron. When they're all dead, move to the southwest then to the north. Follow the path. At this point you don't have any real opponents. If you lose anything, you can replace it. Sapphiron will turn the tide more than any other unit so he's the best target for death coils. Don't move on mindlessly, you should get some reinforcements from your base because the last few battles can be challenging. A good place to stop is the fountain of health.