This next chamber has quite a lot of Nerubians which are no real danger to your heroes but can kill off your other units quickly. As such, use death coil for healing only and have it ready as often as possible. There are more gold coins to be found here, as well as a crypt fiend who will start the second optional quest once rescued. The platform in the next room will drop you into the water as you approach the door with your army. The crabs here are a welcome fight for the experience for Anub'arak and the ring of protection +3 they drop for him. Anub'arak should have well over 50% damage reduction at this point.

The next room, to the east, contains a belt of giant strength +6 that is great for Arthas or Anub. I chose to give it to Arthas, dropping a claws of Attack +3 for it. When you pick up the belt, a large force of Nerubians will attack. Use death coil for healing and try to hit as many creatures as possible with Anub's impale ability. You will have two choices as to how to proceed from here. Take the southern exit to enter a chamber with more trapped crypt fiends. Lasso them and add them to your main group so that Arthas can more easily heal them. They all start a bit damaged, but you shouldn't really lose any if you act quickly. The rolling stone door here takes you where the last door would have led, to a room with a fountain of health. The egg sacks here contain more gold coins for your later pursuits. Heal up at the fountain before you proceed east.

This room can be tricky, but not if you act quickly. Use a crypt fiend and immediately run around the room hitting all four switches. Don't worry about the other units in your army, as stopping the flow of hatchlings is the top priority to prevent you from being overwhelmed. The boss here will drop a medallion of courage, which I took in replacement of Arthas' pendant of energy. The egg sacks here hide a rune of healing, a tome of health (good for Anub'arak with his higher armor) and another stash of gold coins. Proceed through the eastern door. There's nothing to be found in the hatchlings' chambers.

Once you reach the next large chamber, kill the smaller Nerubians first, as the Lord has 2500 hp and you can more quickly remove damage from the battle by killing the weaker opponents. Don't worry about conserving mana too much, as one of them will drop a rune of mana. The room's crates hide the final gold stash along with a tome of intelligence +2 for Arthas. He'll need more mana in the final chapter to heal your units. The next exit is to the west through an iron gate. It leads to the Faceless ones, who infest this entire room. Kill them off one at a time using focused attacks. The last of the captured crypt fiends will be found here. You should have at least a full army of sixteen units by now, making the battle against the Forgotten One to the east pretty trivial. When you first enter his chamber, look for runes of healing and know where to go should one of your heroes get damaged. When you first engage him, he'll summon some helpers. Focus your atacks on the Faceless one regardless. He should fall before long, ending the mission.