Like all psykers of his reality, Cargn is capable of channeling the chaotic energies of the Warp with his mind to create a variety of effects. However, the power of the Warp is difficult to control, and it is also the realm from which Daemons are created. Every time Cargn uses his psychic powers for more than simple, subtle effects, he runs the risk of drawing the attention of daemons in the Warp, who could attempt to possess him or devour his soul, or, in some cases, even create a doorway through which the daemons can enter the physical realm. In addition to this, there is also a chance that the chaotic energies of the Warp channeled through him could "burn" his soul, even destroy it, if he attempts to push his abilities too far.
Cargn's sheer psychic ability place him somewhere between Beta-Primaris and Alpha-Tertius levels, the strongest a Psyker can be without losing control at the slightest use of their power. However, if he calls upon the full extent of the power gifted to him by his Soul-Binding (see below), he can reach levels as high as Alpha-Secundus or Alpha-Primaris for short periods of time (essentially making him capable of an apocalyptic-level event). However, due to the massive risks associated with doing so, Cargn has never yet called upon such power in his entire life, saving such an extreme measure for when a truly dire situation arises.
Soul Binding: Roughly three and a half centuries ago, after having been called to have an audience with the God-Emperor, Cargn was granted the gift of Soul-Binding. Normally reserved for Astropaths--those psykers who use the "light" of the Emperor's soul as a psychic beacon so that interstellar navigation is possible for humanity--this ancient ritual takes a miniscule portion of the Emperor's soul and binds it with the recipient, greatly increasing the psyker's power. It was through this ritual that Cargn became such a truly powerful psyker, and as mentioned above, he can use the piece of the Emperor's soul embeded within him to channel a portion of the Emperor's power, granting him even greater psychic abilities, though at great risk. As with all recipients of Soul-Binding, Cargn's eyes were destroyed in the process, though his psychic powers allow him to see even without them.
Divination*: If there is one psychic discipline Cargn excells at abolve all others, it is the art of divination. Through his connection to the Warp, he is able to see the numerous possible paths fate may take--though it is only through intuition and centuries of experience that he can tell which is the one that will actually occur. Most of the time, Cargn uses this ability to view the present, replacing his lost eyesight. With very slight concentration, he can simultaneously view a couple seconds into the future, allowing him to react to things before they even happen, making him very dangerous in close-combat. Using powerful arcane rituals, however, Cargn can see even further into the future, even things centuries away. However, the further away from the present his visions are, the more vague and difficult to decipher they become. He can also use this ability to "soul-read" an individual. This allows Cargn to almost-literally "see" a person's soul, giving him vague information about their lives. The longer he is around someone, the more detailed a picture he gets from reading that person's soul, and over time can come to understand a great many things about them, though this often takes a great deal of time to accomplish.
*- note: I talked to Saphie about this a while back, and I'm mostly going to be using this in the same manner as Leela's prophecies, in which they provide vague clues about the plot every so often without directly spoiling anything. I'll discuss it with you guys every time before I go ahead and have him do a major reading so that we can decide what to hint at.
Telepathy: Like many psykers, Cargn is capable of sending telepathic messages to other people over great distances. He can send such messages to anyone on or within orbit of the same planet as him (or an equivalent distance in space), though he first needs to be aware of their presence in order to do so. He can easily scan someone's surface thoughts, and with concerted effort, he is capable of reaching even further into someone's mind. Generally, the deeper he goes into someone's mind, the more likely they will be to know what is happening (so he won't be able to steal someone's deepest, darkest secrets without them knowing). He is also capable of psychic suggestion using this power, allowing him to psychically implant a thought or command into someone's mind. Simple commands, particularly those which wouldn't directly bring harm to the target, are easier than more complex or dangerous commands. He can even force someone to forget something for a certain period of time (though not forever). Those with psychic defenses or strong willpower are obviously much less likely to be affected by this ability. Also, the target's thought patterns have to be similar to a human's, so while he would be able to read the mind of or perform psychic suggestion on an alien like a Kryptonian (since they are human-like), he will definitely NOT be able to use it on beings like the Dalgig or the Ohm.
Exorcism: Both through finely honed skill and application of his psychic power, Cargn is capable of banishing daemons and similar outer-dimensional creatures back to where they came. For lesser daemons, this is often simply through sheer force of will, though for more powerful daemons and creatures, it takes much more time and effort. Often, while exorcizing a powerful daemon, he will employ the help of other psykers and spiritually attuned people to aid in the effort (normally fellow Grey Knights, but anyone with similar abilities will do).
Astral Projection: Cargn's immense psychic ability also allows him to extend his consciousness past his own body to produce a variety of effects. The basic use of this is to create an astral form through which his consciousness can act. This often takes the form of an "idealized" form of himself (in his case, a tall man wearing a simple grey robe and a blindfold) which can travel anywhere within a couple miles of his body. This form cannot interact physically with anything without being extremely close to his real body, but he can communicate with others using this power. In addition, if another person has a strong psychic presence, he can pull their consciousnesses from their bodies into an astral form of their own. If he does so against an enemy, he can then fight them in the astral realm. The loser of such a battle goes into an unrecoverable vegitative state, their mind essentially destroyed.