Brain Spider

You choose 3 creatures within range that you have seen and spoken to at least once before. The selected creatures must make a Wisdom saving throw. They must make the save at disadvantage if you know their names. Creatures that make the save resist all effects and are immune to this spell for 24 hours. Those that fail the saving throw suffer the spell's affects. The creature is not aware of the spell even if they pass the original saving throw. 

For the duration of the spell, as an action, you can eavesdrop on the thoughts, sight, and hearing of the creatures affected by the spell. In combat, once per round you can attempt to discern a weakness of an affected creature (vulnerability, resistance, immunity, AC, current hit points). This takes a bonus action. The creature can make another Wisdom saving throw to resist the intrusion. Success on this saving throw does not negate the other effects of the brain spider for that creature.

At the end of the duration of the spell, either by time or the caster ending the spell purposefully, the user may attempt to detonate the brain spider within the affected creature's mind, dealing 8d10 Psychic damage. A creature may make a Wisdom saving throw to avoid this detonation. If a creature's brain spider is detonated or they pass the saving throw to avoid detonation, they will become aware of the spell that was cast on them, who cast the spell, and where the caster's exact location is.