Homebrew
Dominate Person: Once the red leech has burrowed into its victim and reached the victims brain it takes one turn for the leech to latch onto the victims brain. Once this attachment is complete, the victim suffers the effects of the Dominate Person spell (PHB pg. 235) falling under the control of the High Priest of the Drastikan who created the leech. The is no savings throw that can mitigate the effects of Dominate Person spell and its duration last until the red leech is killed. The only way for the red leech to be removed or killed is for the victim to be targeted by an effect that cures disease or removes a curse.
Burrow: The red leech can burrow through the soft tissue of a target. It takes two turns for the leech to completly burrow into the target. The target takes 2 points of necrotic damage for each turn. In that time the leech can be removed from the target. If the leech completely burrows into the victim the only way for it to be removed or killed is for the victim to be targeted by an effect that cures disease or removes a curse. The red leech will continue to burrow through a victim at a rate of one foot per turn until it reaches the victims brain. Each turn a leech burrows the victim takes 1 point of necrotic damage.