You focus psychic energy to a target you can see within range, the target is forced to roll an Intelligence Saving Throw. On a failed save the spell locks on to the target and takes effect (described below). On a successful save the target takes 8d8 Psychic Damage as it is overwhelmed by psychic energy.

On a failed save the target takes 2d8 Psychic damage and its intelligence is reduced by 1d4. The target may repeat their save at the end of each of their subsequent turns while concentration is maintained. On a successful save they still take 2d8 Psychic damage, but their intelligence is not reduced, on a failed save the target's intelligence is reduced by 1d4 and takes 4d8 Psychic Damage. 
When a target's intelligence is reduced in this way it cannot be increased by any means other than a wish spell, divine intervention, or a greater restoration spell cast at 9th level. 

The effect ends on 3 successful saves or if the caster is ever more than 300ft away from the target for a full round. 

After 3 failed saves the target's intelligence is permanently reduced and the creature acts like a thrall under your command. A thrall keeps its original statistics but may only regain half of its maximum hitpoints per long rest, and may only regain spell slots if another caster chooses to put spell slots into them of a level the thrall could cast. (i.e. if a wizard puts a 3rd level spell into the thrall, they regain a 3rd level spell). A creature with an innate spellcasting feature would regain their spells as normal without this requirement. A thrall will follow your commands to the best of their ability, even if such commands would risk its own life. If no commands are issued the thrall will prioritise defending you, and then itself.

You may only have a number of thralls equal to your Spellcasting Ability Modifier at any one time

If a creature's hitpoints or intelligence modifier is reduced to zero by this spell the creature is considered to be dominated by you as if by a Dominate Monster spell for the next 24 hours. A creature Dominated in this way regains its intelligence score and half its maximum hitpoints immediately, and dies at the end of the 24 hour period. 

*Attempting to cast this spell on a creature without a brain will result in the failure of the spell without the material components being consumed.

**Material Components will only be consumed if the creature fails on its initial save