Deep within Grimrock lies something that is imprisoned, something that is asleep, something broken. A terrible power, a power so evil it nearly destroyed the world thousands and thousands of years ago, his crimes forgotten but never forgiven. For within Grimrock lies something that must not be freed, must not be awaken, must not be whole again. And if the work of the Designers become undone, all life will be destroyed. For within Grimrock lies the ‘Undying One’, eternally imprisoned to depths beyond measure. . .
The Undying One (Named Cube in the editor) is found in level 12. He is the final boss, a large mechanical cube, and the prisoner of the dungeon of Grimrock. He is the one who has communicated with the player through dreams. He was imprisoned there by The Designers and some of his parts were ripped from his body. He can only be destroyed with the help of the Weapon of Power.
The Undying One is activated when you return his four missing pieces found in level 13 - Ore , an Infusor , a Bladed Gear and a Steel Gear - to his body, located where the Wardens had been. Revealing that he has been manipulating you into repairing him, he will then attempt to kill you, and you will be unable to harm him. At this point, a teleporter will open in the southeast corner of the map which leads to the part of level 11 housing the Weapon of Power. The Weapon must then be used on the Undying One to stun him and remove the four pieces earlier reinserted by the player; afterward, the Undying One will become vulnerable to normal attacks.
During the boss fight, the Undying One will continually spawn enemies, including Scavengers , Ice Lizards , Uggardians , Goromorgs and Shrakk Torrs . This makes the battle much more difficult, both because they can harm you and because they can get in your way as you attempt to escape the Undying One, who will instantly kill your team if he manages to crush you.
The Undying One is weak against electrical attacks; for this reason, locked rooms on level 12 contain Lightning Bombs and a Lightning Rod . These rooms can be accessed via the use of keys randomly dropped by the monsters spawned by the Undying One. The Undying One can also be attacked with the Weapon of Power, which has an unlimited amount of charge.
The Dismantler's name is a reference to the Undying One, given its chance to inflict shock damage (however, it is usually risky to engage the Undying in melee combat, reducing the Dismantler's effectiveness).
The Undying One can be seen in the background of two shots of the "true ending" outro of Legend of Grimrock II, standing still along with statues from Mount Grimrock in a hall with treasures and books.  He can also be dug up in the Barren Desert as a part of the ultimate secret.  However it is not known if this is meant to be just an Easter egg or an actual part of the lore to be taken seriously.
Talking to player Edit
The Undying One starts talking to the player telepathically early in the game when they go asleep. He pretends to be their ally wanting to escape the mountain with them. He continues talking to them when they are asleep. As they get close enough to him, he starts talking to them even when they are awake. The following table lists the lines spoken by The Undying One.
What is happening?
A collective dream!
Oh, I know you.
I know what you seek.
Look for me down below, and I might help you.
Are you still there?
I know a way out but I can't make it there by myself.
We need to work together.
Descend towards the bottom of the mountain. We can meet there.
|I can't hear you but I know you can hear me.
I tried to talk to the other people that have been here before you.
They were criminals but I sense you I can trust.
|I watched this dungeon being built.
But they took it away from my people and twisted it.
Filled it with traps and riddles.
That is not how this place was meant to be.
|They make all the gates open and cause the traps to spring.
It would be simple for them to stop us.
Why will they not do it?
Are we walking into another trap of theirs?
|They keep all the machinery and mechanisms working in the dungeon.
All but one that sits in the tunnels below.
It needs to be repaired so we can leave this place.
Only a little bit further.
You need to be careful.
They live here and they will want to stop you.
They fear you might undo their vision.
|The broken mechanism controls the Portal.
From there we can escape.
I am still too weak to move but I am glad you are making progress.
I need to gather my strength so we can leave together.
|You are close enough now.
I don't have to wait for you to be asleep anymore.
A long time ago they closed the Portal by breaking the mechanism.
It is here, nearby. Find it.
|First time talking to the player when they are awake.|
|They throw all their scrap metal and old parts down the pits.
We may find what we need here.
|They destroyed the mechanism.
They took it apart to keep the Portal closed.
But it can still be mended. I know how.
To reopen the Portal, you need to find the missing parts.
There are four parts missing. Go.
|Aaahh... I am whole again.
You fool. You only lived so I can be set free.
The only freedom you shall have is death!
|Spoken directly when The Undying One is repaired.|
|Stop struggling.||Spoken in battle with the player.|
|Stop it!||Spoken in battle with the player.|
|This can not be happening.||Spoken in battle with the player.|
|Aaargh!||Spoken in battle with the player.|
|No! This can't be happening!||Spoken in battle with the player.|
|Aaaaarrgh!||Spoken in battle with the player.|