Epsilon.
How high is this rank in the psyker hierarchy to provoke so much doubt in the hearts of the Astartes under Atreus's orders?
To understand their fear, one must first look at the classification of psykers within the Imperium.
Psykers are divided into different levels of power. Untouchables are not included in this classification, as they possess negative psychic value.
Unlike ordinary psykers, Untouchables do not shine in the Immaterium.
They are more akin to psychic "black holes."
Thus, officially recognized psykers are classified according to the following ranks:
Rank Rho (ρ)
Description: Barely perceptible psychic sensitivity, heightened intuitions and premonitory dreams.
Danger level: Low.
Rank Pi (π)
Description: Limited telepathy and minor extrasensory perceptions.
Danger level: Very low.
Rank Omicron (ο)
Description: Minor telekinesis, superficial mind reading and weak Warp manifestations.
Danger level: Low.
Rank Xi (ξ)
Description: Object control, limited mental influence and elementary biomancy.
Danger level: Moderate.
Rank Nu (ν)
Description: Level of a competent military psyker. Notable telekinesis, pyromancy and advanced telepathy.
Danger level: Dangerous.
Rank Mu (μ)
Description: Capable of facing multiple opponents simultaneously, launching Warp lightning and creating psychic barriers.
Danger level: Very dangerous.
Rank Lambda (λ)
Description: Powers sufficient to destroy light vehicles, control multiple minds or provoke localized psychic storms.
Danger level: Extreme.
Rank Kappa (κ)
Description: Comparable to certain Space Marine Librarians or exceptional Inquisitors. A loss of control can lead to the annihilation of an entire company.
Danger level: Catastrophic.
Rank Iota (ι)
Description: Extremely rare level. Massive Warp manipulation, teleportation and destruction of entire buildings.
Danger level: Cataclysmic.
Rank Theta (θ)
Description: Capable of transforming a battlefield alone, summoning or banishing major daemons.
Danger level: Strategic weapon.
Rank Eta (η)
Description: Powers capable of affecting entire cities. Very few psykers of this level survive long without succumbing to corruption.
Danger level: Major disaster.
Rank Zeta (ζ)
Description: Significant alteration of local reality, Warp storms and large-scale destruction.
Danger level: Apocalyptic.
Rank Epsilon (ε)
Description: Potentially planetary threat. The Imperium would immediately seek to execute or contain such an individual.
Danger level: An Exterminatus may be considered.
Rank Delta (δ)
Description: Psychic control of thousands of individuals and destruction of entire cities.
Danger level: Planetary.
Rank Gamma (γ)
Description: Individuals recorded at this level are practically mythical.
Danger level: Solar system.
Rank Beta (β)
Description: Can rival, depending on circumstances, certain Daemon Princes or Greater Daemons.
Danger level: Sectoral.
Rank Alpha (α)
Description: The most powerful psykers officially listed. They can cause the fall of entire worlds, open immense Warp rifts or annihilate armies.
Danger level: Segmentum.
Rank Alpha Plus (α+)
Description: Practically immeasurable power. These beings can distort reality, destroy planets or provoke major Warp catastrophes.
Danger level: Civilizational threat, even existential.
By observing the classification of psykers within the Imperium, it is easy to guess the fate reserved for someone like Loki in a world under the imperial yoke.
He represents a sufficiently important threat for the Administratum to consider an Exterminatus in order to eliminate any risk.
It was precisely for this reason that Atreus had, until now, sealed Loki's psychic potential, after having evaluated its limits by deliberately provoking the death of his pet.
Was it a trauma?
Possible.
However, if his child was unable to overcome such a loss, then he would be even more vulnerable in the face of the disappearance of a loved one.
"Was he always this weak?" thought Loki while manipulating the energy of the Immaterium to deflect the attacks.
The frail arms of the eleven-year-old boy repelled the intimidating assaults of the Catachan Devil, while his legs made him move around the xenos creature with surprising agility.
He did not know where this confidence came from.
Yet, he had the impression that the energy flowing through his body guided each of his movements, instinctively directing him toward the best attack angles during his movements from tree to tree.
"GRAAAH!!"
The beast let out a roar of rage after Loki's blades severed several of its legs.
Its eyes filled with infinite hatred.
Yet, despite this berserk frenzy, the outcome of the fight no longer seemed in any doubt in the eyes of all the beings present.
The thick carapace of the Catachan Devil had represented an insurmountable challenge for Alexandre and his half-brother before Loki's return.
But in the face of the Immaterium, it looked like a simple layer of butter.
With an acrobatic leap from a tree branch, Loki propelled himself toward the creature's head without the slightest hesitation.
"O-yes… I can do it," he muttered while letting himself be guided by the flow of the Immaterium.
The young boy could feel it.
The beast's fear.
The anxiety of his companions.
The vibrations of the air.
The doubts present in his words.
All of this swirled in the Immaterium, forming an endless maelstrom in his mind still inexperienced in the esoteric arts.
He had the impression of knowing everything.
"Uh… L-Loki is smiling?" uttered Alexandre while observing the boy slowly approaching the monster's vital points.
Indeed, Loki was smiling.
Like a person who had finally understood an obvious truth, he felt something change within him.
For the first time, he felt truly "complete."
In this state of epiphany, Loki better understood the true nature of his power.
Illusion.
Several silhouettes of Loki appeared in the air.
Previously, he had simply been leaping from tree to tree after counterattacking the beast.
But that was no longer the case.
Dozens of copies of himself simultaneously assailed the beast.
The fight, already one-sided, ended much faster than Atreus had anticipated from the planet's orbit.
Slash.
"Gaaah…"
A groan of pain escaped from the creature, before giving way to Loki's labored breathing, mixed with an uncontrolled laugh.
"Ah… ha ha… this power…" he murmured while pushing back his hair stained with the Catachan Devil's blood.
The innocent child had just been baptized by the blood of the Xenos.
He was no longer simply Loki Polus.
He was now a warrior.
"It is really mine…" he muttered before abruptly interrupting himself, bringing a hand to his head.
The confidence that had inhabited him a few seconds earlier had disappeared.
Loki lost his balance.
An intense pain crossed his mind, the inevitable consequence of using a power exceeding his limits. Blood even began to flow from his eyes and ears for an unknown reason.
Loki had used a force far superior to what his body could withstand.
An inevitable consequence.
An ordinary Space Marine psyker was generally around the Kappa rank.
But Loki had just brushed against the power of a true Epsilon-rank psyker with a simple mortal body.
An anomaly.
Without Atreus's seal, the boy would probably have become a Warp portal long ago.
"I-I must not… I…"
As he struggled to remain conscious, an unknown force suddenly invaded him, slowly dragging him toward sleep.
Loki's vision gradually became blurry while his companions rushed toward him as fast as possible.
The last thing the boy had the impression of seeing was a massive silhouette, which slowly nodded in his direction.
**Author's note:** Finally, I finish this damn Odyssey in the next chapter (today).
And if you want to know where the Emperor, Malcador and Magnus stand in the psyker classification, it's simple: Alpha+.
As for characters like Mephiston of the Blood Angels, I haven't found a precise rank. However, he is officially considered close to the Beta level, which already places him above an Epsilon-rank psyker.
Besides that, the danger level does not necessarily represent the raw "power" of a psyker, but rather the threat he represents to the Materium if he were ever to lose control or succumb to Chaos.
For example, if the Emperor had accepted the idea of a Chaos-linked divine ascension during his confrontation with Horus, he could have become an existential threat comparable to Slaanesh (in a way a fifth Chaos God).
Thinking about it, Magnus probably wasted part of his potential by becoming a Daemon Prince. Despite all his power, a Daemon Prince remains bound to a Chaos deity and is not totally independent.