Arcane Smell. The Mana Leech has advantage on Wisdom (Perception) checks to find magical creatures

Magic Dampener. In a 15ft radius around the Mana Leech, magic is difficult to cast. To cast a spell while within this radius, the caster must make a DC 14 (Arcana) intelligence check. on a failed check, the casting of the spell fails.

Spell Thief. The Mana Leech's Spell casting ability is Wisdom (DC 14) however the Mana Leech cannot cast spells on its own. it must use its drain action on other spell casters to gain spell points to be able to cast spells. the Mana Leech can cast the following spells with no components:

-2 Spell Points: Shield
-3 Spell Points: Darkness, Hold Person
-7 Spell points: Greater invisibility at 5th level

Multiattack. The Mana Leech makes one bite attack and then another with its drain 

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 14 (2d10 + 3) Piercing damage. The target is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the Mana Leech can't bite another target.

Drain. The Mana Leech drains the arcane power of it's victim empowering the leech in the process. Drain deals (22) 4d10 Psychic damage to a target restrained by the Mana Leech. The target then loses a 1d4 level spell slot, if the target is a warlock it takes the targets maximum level spell slot. The Mana Leech gains Spell points equal to the level of the spell slot stolen. If the [psychic] damage reduces the target to 0 hit points, the target is stable but unconscious for 1 day, even after regaining hit points. This ability can only be used against the target that has been restrained from its bite.