In Warhammer 40k there is no magic like you find in the Age of Sigma game. What you have instead psychic powers.
Psychic powers can only be manifested by special individuals called psykers (duh!). All psykers can manifest a psychic power called smite.
These note are written with the expectation that you have the Warhammer 40000 8th edition rule book, or you have regular access to one.
Chance of Manifesting Psychic Powers
To cast a psychic power you need to roll 2D6 and get a result equal or higher than the power level of the power. You need to pick the power before you roll, so you need to ballance the chance of success with the result you need. If two powers will give you the result you want, for example the death of a character, then use the power that will be easiest to invoke.
All psykers get the Smite psychic power, so you need to get familiar with it. Even if you have no psykers you are likely to be on the receiving end of it.
Smite has a psychic level of 5. This means that it is easy to cast. However the psyker needs to be within 18″ of the target, which is well within counter assault range. This short range means that your psykers will need some body guards.
With Smite you can only target the nearest visible enemy unit. If you are in 1″ of the enemy then this should be easy. If you are touching 2 or more units you can choose which of these units you want to smite. If you are further away you may want to use the movement phase to get in position to Smite your preferred target.
If you get the Smite power off then it causes D3 mortal wounds to the target enemy unit and if you rolled 10+ then you get D6 mortal wounds. Mortal wounds carry over, so this can, and often does, remove several models from the target unit. The best target units are those with only a few wounds ( or only a few left) that have high save and invulnerable saves. Terminator Squads or Characters are my favourite targets.
Deny The Witch
The main use of psykers is to cause damage to your enemy or strengthen your army. However there is another very important role for your psykers, and that is to block successful psychic attacks from enemy psykers. This blocking action is called ‘deny the witch’.
Deny the witch rolls can be made 24″ away from the enemy psyker. Notice that this is further than the 18″ range of Smite. This means that you can keep your psyker a bit further back. Note though that other army specific psychic powers may have a longer range!
Army Specific Psychic Powers
Each army has, or will have their own Codex which include special psychic powers that are available for that army only. I will be covering these powers in the general tactics section for each army.