Amphibious: The Aboleth Leech can breathe on land and in water.

Mucus: The Aboleth Leech has the ability to produce harmful mucus. The Leech has 4 abilities involving mucus, and each of these abilities cost a certain amount of points to use. Once all of the points are used up, the leech dries up. The Leech must be submerged in water for 30 minutes to gain 1 point back. The Leech starts with 6 points. The abilities cost 1-3 points to use. Some of the abilities are actions, while others are bonus actions. The leech must have enough points left to use an ability, if it does not, the Leech cannot use that ability. The Leech can never exceed its maximum points. The abilities are listed below:

• Mucus Cloud(3 points) The Aboleth Leech creates a cloud of mucus around itself. The cloud stretches 20ft in all directions, originating from the leech. Any creature that starts its turn in the cloud must succeed on DC 15 constitution saving throw or become diseased, and take 1d4 acid damage. A target is diseased for 1 hour. While diseased, the target can only breathe underwater. The cloud lasts for one minute. In addition, the area is heavily obscured. This costs an action 
• Mucus Trail(1 point) The Aboleth Leech launches itself 30ft in any direction it chooses. This movement does not count towards the leeches regular movement speed. A trail of mucus follows the leech, and mucus fills any space the leech went through while it launched. The mucus only fills spaces the leech launches through, not spaces it moves through after. This Mucus stays for 1 minute, and any creature that comes into contact with it takes 2d6 acid damage. This is a bonus action.
• Mucus Coating(2 points) The Aboleth leech covers itself with a thick coating of slimy mucus. This mucus stays on the Leech until the end of its next turn. While covered in mucus, all melee attacks against the leech have disadvantage. The leech also has resistance to Bludgeoning, Piercing, and Slashing damage, In addition, the Leech has advantage on dexterity saving throws. If a creature touches the leech while the mucus is present, they take 1d6 acid damage. This is a bonus action.
• Mucus Bite(2 points) The Aboleth Leech fills its mouth with mucus, as it prepares to attach with a host. If the Aboleth Leech hits a bite attack within 1 minute of using this ability, the bite deals an extra 3d6 acid damage, and the target must make a DC 15 constitution saving throw or become diseased for 1 hour. A diseased target can only breathe underwater. This mucus stays even if the leech misses a bite. This is a bonus action. 

Bite: Melee Weapon Attack: +8 to hit, reach 5ft., one target. Hit 1d8+6 piercing damage plus 1d8 acid damage. The Leech automatically attaches to the target. By attaching to the target, the Leech gains access to its control, life drain, and physic spike abilities. Once the leech is attached to someone, it cannot attach to someone else. Unattaching from a target costs an action. If an attack against the leech misses, it hits the host instead. While attached the Leech can make a life drain attack as a bonus action. The Leech can then use it's action to make another life drain attack, use it's control ability, use it's physic spike ability, use it's mucus cloud, or detach from the target. The Leech can choose to use one of it's mucus abilities for it's bonus action instead of making the extra life drain attack.

Life Drain: The Aboleth Leech drains the life of its host, and then uses that life to replenish its hitpoints. This attack can only be used if the Leech is attached to its host through its bite attack. The target takes 2d6+3 necrotic damage, and the Leech regains hit points equal to the damage dealt. The leech cannot exceed its hit point maximum using this ability.

Physic spike: The Leech sends a spike of physic force to the brain of its host. The leech must be attached to its host through its bite attack to use this ability. The host takes 3d12 psychic damage and must succeed on a DC 16 constitution savings throw or become paralyzed. 

Control: The Leech attempts to control its host. The Leech must be attached to its host through its bite attack to use this ability. The host must succeed on a DC 17 wisdom saving throw or be under the leeches control during its next turn. The leech controls all actions, movement, and anything else the host would do during its turn.