In the world map is a level 5 camp with the name "Adamant's Fortress".
If you collect informations via Riz about "Adamant the creator" you found a hidden entrance.
Its a fight with two parts.

The first part is a fight were the units and the soldiers are splitted (or did I miss something in the placement phase?). Just reach the two lever with your soldiers (better to take swordsmen). The Gladiators fighting in the upper part of the level and help with range/magic to reach the levers.

After the team reaches the portal (are the green exit markers missing?) the second part is started. You will face Adamant the creator in a kind of puzzle level.
It takes me a long time to get behind the mechanics and I have to restart it multiple times. That's why I create this guide.

On the map you have only two enemies. "Adamant the creator" and and sandghost with 50 live.
The map shape is like a crossroad with 6 directions with a blocking sphere in middle and a circle way around.

The way is blocked with multiple pairs of coloured spikes.
4 Coloured Levers to open the spikes, but there can only one active. Activate one will close all other spikes.
5 Crystals how heal the enemies.

Two Enemies, one of them a sandghost? looks easy, in the last level we blast three of them away. But here we have multiple crystals who instantly heal the sand ghost and the mage to full life. If we somehow kill the sandghost (burst damage or trick it on a spike), it come back to the fight with full hp.
So what can we do?

There can be one active lever who opens the spikes. We need to activate the red and blue switches multiple times to pass every spike and reach the cristals.
If all 5 crystals are broken, all spikes will open and adamant will start the final fight.

The sand ghost
The sand ghost acts different then the other ghosts before.

a) It will remain on his position and attack you with a kind of "Wrath of the elemental" spell. Luckily it often miss.
b) If it has the opportunity to strike your gladiators in a even line, it will change his position and bring them to ground.
It looks like the position swap is forced if at least one gladiator is at the other side of the line, so they try to divide them.
c) If gladiators in close range it attack then with melee, this can "eat" then! The gladiator is out of combat till the sandghost is changing position again. This is the same like characters trapped under creates.

If killed it respawn again. I think this stop after playing the music in the top right desk.

Adamant the creator
The yellow mage makes the show interesting.

Phase 1) He stand in the top starting position and only move 1-2 step to attack in close combat. Usually he casts spells.
a) A level 4 "Aching Limps" spell drawn all your stamina and slow your heroes.
b) If one or more heroes near the map centre he cast a level 4 "Ball of lighting" in the middle of the map. This spell can really hurt your units. Very hard combination to the sandghost ability. Best is don't try to trigger it. If one of your chars is injured/low health, try to go closer with an other char to tank the damage. The cloud will always attack the closest char, this can be the sand ghost.
c) If one character is near a outer crystal (not C1) the cast a level 3 "Frency" on the char, what usually delays everything for hours.
d) If you attack him in close combat, he takes his poisoned monster cleave and attacks you with a huge amount of damage. (just stay away from him)
e) He drinks mana potions (don't know if this is limited)

Phase 2) Starts after all crystals are destroyed. He teleports in the middle and cast an high level "Fastness of body" on himself. Now he waits till close combat is possible (think now its possible to kill him from distance).

After we know the mechanics you can easily find a way to solve this "puzzle fight". For me it was very difficult to finish this fight. I think later in game, with better equipment and higher skills its not really easier.

Here is my way to finish this one:
a) Starting with 2 chars running up. Trying to hide in the left or right side of the middle (to avoid receiving the sandghost attack). It is important that the sand ghost switches his position. Otherwise he blocks your path.
Keep your healer out of combat and heal every incoming damage.
I runned to the top, open one lever (i usually start with the blue one, it doesn't matter) and run with one char behind the spikes (puh).

b) Then smash the crystal and run with one char down to the second spike group (if you start with blue, open the red, this closes blue) and reach the other crystal.

c) Adamant will cast frenzy on the first char how reaches the second crystal. So its better to trap this char in one side of the map and move with 2 other chars in the other side of the map.
Destroying the crystal will take some turns (heavy blow attack helps, don't use weapons with magic damage), its important to keep one gladiator in the near of the top levers, keep him alive and stay away from the mage, otherwise he will hit you with his poisoned monster cleave.

d) After destroy the second crystal the top lever must pulled one additional time.
Destroy the third crystal and pull the lever to swap the side. Be careful with the map middle to not cause the "Ball of lighting" spell.

e) Reach the other side and destroy the third crystal. In this moment Adamant stops his frenzy cast (for me), so the last one was easy.
Now every spike (even the uncoloured) are open and adamant teleports himself in the map middle.
Now I killed the sand ghost first and then slowly get in position to the final fight. always watch to keep my health is high.

Other:
A user speaks about beaten adamant in 5 turns http://psteamcommunity.yuanyoumao.com/app/314830/discussions/0/611696927914321247/

Everyone how win this hard fight can say hallo to the boots/ trousers / robe of the yellow king and a magican staff (2handed, vitality +5, astral energy +5, astral regneration +1).