Retrieve the Crystal of Terrax is a promotion quest in Might and Magic VI: The Mandate of Heaven. Completing this quest will promote all Wizards in the party to Arch mage, giving them one extra hit point and spell point per level.
Lord Albert Newton tells you that he can't instruct you in the knowledge you need to become an Arch Mage unless he has the Crystal of Terrax. Actually, he doesn't say that much since he can't remember the name of what he needs, but he does tell you to search Corlagon's Estate for the whatever.
Monsters[]
Cutpurse → Bounty hunter → Assassin
Follower of Baa → Mystic of Baa → Fanatic of Baa
Lizard man → Lizard archer → Lizard wizard
Skeleton → Skeleton knight → Skeleton lord
Ghost → Evil spirit → Specter
Walkthrough[]
Albert Newton can be found in Castle Newton, in Mist. Corlagon's Estate can be found in the far southwest of the Castle Ironfist region, on the coast. You can find out the name of what you are looking for (the "Crystal of Terrax") by looking at your quest log, since the magic writing knew what Lord Newton meant even if he couldn't immediately recall the name himself.
Once inside the estate, you will be confronted with two doors. Choose the right-hand door. It may take one or two turns, but eventually you will find yourself in a long hallway with passages leading off to the right and left. Take the first passage on your right and open the door. Continue through that room and down the corridor, following it until you come to another door.
Behind this second door is a sunken room full of monsters (some of whom can float at your level). There is a catwalk you must negotiate to get to the next door. This catwalk goes straight out to the middle of the room, makes a sharp left turn, continues for awhile, then makes another sharp left turn before getting you in front of the exit doorway (this exit is on the same wall as the door from which you entered).
Once you are through the doorway, when you turn the corner you will see some monsters ahead of you; however, there is a teleporter in the middle of the hallway that will transport you to the end so that they will end up behind you.
You should now be in a hallway with a couple of short dead-end passages. You might notice that these are crypts, and that partial walls block access to them (this is important later). For now, go straight to the end of the hallway. Open the door and go through the opposite door, continuing on, across a bridge to the end of the hall.
Go on to the end room where you will find a switch in a wall. Press it. Now, return to the bridge, cast Feather Fall, and jump down. Two doors lead out of this room; take the south door. Go back up the corridor until you come to another long hallway with a lot of short dead-end passages (crypts), with one exception. Pass the exception (there is nothing remarkable in there) until you get to the short passage that is a little longer than the others. There is not a crypt here, but a switch in the wall. Unfortunately, it is still behind one of those partial walls so that you can't reach it. If someone in the party has the Telekinesis spell, you can shortcut around the next part by just using it to press the switch. Otherwise you will need to find the secondary switch to lower the wall.
To lower the wall: Continue to the end of the hallway and enter the next room. In it is an (empty) well. Cast Feather Fall again if it is not still active, and jump down into it. Follow the (long) corridor along until it makes a right turn. Then go down to the first left, and press the switch on the end wall. Come back out of that alcove with the switch, and turn left (not back the way you came), and follow the corridor to the end. Turn left and follow it around until you come to a door. Open it. You should now be in a room with a (full) well. From here, take the path back to the entrance, pass through the entrance area through the left door, and follow the ramped corridor down until it ends in the room with the well that you jumped down. Go through the room, back to the previously blocked switch. The wall should now be down and you can press it.
After the switch has been pressed: Return to the room with the bridge from which you jumped. A previously hidden secret door in there should now be open. Go through it to the end room. The Crystal of Terrax is in a chest inside.
Return with the crystal to Albert Newton in Mist for your reward and promotion.