Step of the Ruin

The student could only watch as her professor was facing against the Doubled Witch. She couldn't believe it. He looked like a Gray, and yet acted nothing like one. Instead of cold emotionless behavior, he was joking and smiling as he toyed with the Witch. And why would a Gray even fight a Witch in the first place?
As he jumped back, avoiding one of her attacks, Tanya took the place of her sister and lept after him, forming a sword made of Color in her hand. The blade grazed the air in front of him.
"That's a great trick. Next time try to form the blade a little later. It could catch your enemy by surprise," he barely finished the sentence when another attack came his way. A quick movement of his wrist and it got stopped by his weapon. Tanya pulled her arm and swung again. And again. Whenever the two blades crossed, the professor's sword started getting saturated, only to lose all color thanks to him being Gray.
Lena stepped out of her sister's shadow replacing her. Her attacks were faster but weaker. Even then, the onslaught forced the professor to step back. She moved after him, stepping in the same place he just stood.
"I'm sorry, Lena," he said, smiling sadly. "I'll have to finish our fight for now." His free hand moved, making some gesture. Some of the Blue Color rose up around her. The ground under her started crumbling and turning to sand. Her feet were stuck.
Tanya showed up again, just a step behind where her sister just was. She moved around the affected area, but the ground gave up under her just like it did under Lena. She noticed that the whole section of the building between her and the professor started turning Gray.

The so-called Step of the Ruin is a one-of-a-kind spell, invented and used by only one person, the Gray mage. The spell can only be used by someone who became a Gray and still somehow retained their magic capabilities.

Mechanics of the spell

Requirement #1 - Gray draining

Wherever they are, if Grays stay in one spot for too long, their bodies start to suck out the Colors from the environment. Or rather, the Colors from the environment undergo diffusion and try to equalize the difference of the Color levels outside and inside a Gray. However, since said levels will always be at "0" inside their bodies, it results in Colors being drained around them. Depending on the species of the Gray, this process can be quite fast or very slow. There are some Grays who can speed it up at will.

Requirement #2 - Control over Colors

All of the casters using the Color magic can manipulate at least one Color. While the ones that haven't been educated in this art tend to use their own Colors and burn themselves out and the beginners send the Colors through the Color nests inside their bodies, the true masters can use the Colors that never come in contact with their bodies.

Mixing the two

To prepare the Step of the Ruin, someone who is a Gray stays in one place. Just as they are about to completely drain the Color from the spot, they use their control over it to leave a small portion of it there. Usually, some Blue is added to the affected area since its illusory properties can mask the dim tones. At this point, the spell is ready.

Activation of the spell

From that point on, the Gray caster can at any time call the remaining Colors to themselves, completely draining them from the targetted area.

Gestures & Ritual

Whenever the Gray mage activates the spell, he stretches his arm towards the areas which mask the Grayness and gestures with his hand as if he tried to call something to come to him.
While preparing the spell the caster must personally visit the edges and the longest diameter of the target area.

Effect Duration

Unless restored by a Witch, Gray areas will remain Gray forever.

Effect Casting Time

Between a few seconds and a few months depending on affected area.

Range

If Gray mage's claim that he was the one who destroyed The Kingdom of Fimra is true, then the spell can cover area of a nation. Though for now, his words are heavily doubted.

Effect

Visual:

When the spell is activated, the remaining Colors float up from the target area towards the caster in a form of long (mostly blue) strings. As the strings touch their body, they sizzle and disappear.

Practical:

The spell creates spots or areas completely drained from all of the Colors. Most of the surfaces affected by it become brittle, lose most of their properties, and turn to dust. Depending on the area affected, it can even easily destroy buildings.
The spell can affect plants, fungi, and dead animals, but not the living ones. Still, remaining in the affected area causes normal symptoms of staying in a Gray area (slow draining of one's Colors).

Professor looked back at Tanya, halfway buried in the sand.
"I'm sorry, Doubled Witch. Maybe one day the two of you will understand what future I'm trying to accomplish here."
Hearing his words, Tanya spat towards him. His corners of the mouth turned downwards, turning a smile into an unamused grimace, "Well, at least your sister may."
The professor then turned to where his student was. He bowed and smiled again and said, "I guess you'll have to tell your classmates your teacher is going on an indefinite leave. Well then, bye".
Saying those words, he disappeared.