Arcane Vampire Slime

Amorphous.  The Slime can move through a space as narrow as 1 inch wide without squeezing.

False Appearance.  While the Slime remains motionless, it is indistinguishable from an oily pool or wet rock.

Sense Magic.  The Slime senses magic within 120 feet of it at will. This trait otherwise works like the Detect magic spell but isn't itself magical.

Magic Eater.  The Slime stores magic energy inside itself. Any magical weapon that hits the ooze is affected unless it is sentient or an Artifact. The creature holding the weapon must make a DC 14 Charisma saving throw or have the magic property removed from the weapon for 1 minute. If the weapon's magic property is removed in this way it becomes nonmagical for the 1 minute it has its magic removed. 
Once per turn when the Slime make a Pseudopod attack against a creature wearing magical armour it can force that creature to make a DC 14 Charisma saving throw or have its armour's magic property removed for 1 minute unless it is sentient or an Artifact. If the armour's magic property is removed in this way it becomes nonmagical for the 1 minute it has its magic removed. 

Multiattack. As an action, it can make two attacks with its pseudopods.

Pseudopod. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 28 (8d6) acid damage. The Slime can expend a charge of its magic eater to deal an additional 11 (2d10) force damage.

Magic burst (recharge 5-6).  The Slime emits a burst of energy each creature within 30 feet of the Slime must make a DC 16 Dexterity saving throw, taking 42 (12d6) force damage on a failed save, or half as much on a successful one. The DC of this attack increases for each magic item's energy that Slime has consumed with its Magic Eater feature.

Swallow Magic (recharge 5-6).  When the Slime is forced to make a saving throw or hit by a spell it can use its reaction to consume the spell ending its effect instantly.