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.
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. 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!