Ooze, Arcane (5e Creature)
From D&D Wiki
Large ooze, unaligned
Damage Immunities acid, lightning, slashing
Arcane Absorption. Whenever the ooze is subjected to damage caused by magic, it takes no damage and instead regains a number of hit points equal to the damage dealt.
Amorphous. The ooze can move through a space as narrow as 1 inch wide without squeezing.
Silence Magic. Any spellcaster that the ooze hits loses the ability to cast any spell till the start of its next turn; furthermore, roll a d6. On a 1 to 5, the target is unaffected. On a 6, the target loses a single spell slot chosen from its lowest available at the time.
Lightning Haste. Whenever the ooze is subjected to lightning damage, it takes no damage and instead is subjected to the haste spell till the end of its next turn.
Limited Magic Immunity. The ooze is immune to spells of 2nd level or lower unless it wishes to be affected. It has advantage on saving throws against all other spells and magical effects.
Multiattack. The ooze makes two attacks with its pseudopod attack.
Pseudopod. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) bludgeoning damage plus 10 (3d6) acid damage.
Engulf. The ooze moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces. Whenever the ooze enters a creature's space, the creature must make a DC 15 Dexterity saving throw.
"The creature resembled a fluid mass of green protoplasm. Within its immense bulk, glowing veins throb and pulse with eldritch light."
Created in ancient times, arcane oozes have long since outlived their original purpose and their long-forgotten creators. The bizarre creatures have a strange affinity for arcane magic. Immune to most arcane magic, arcane oozes drain arcane energy from nearby spellcasters. Some speculate that these oozes were created to combat a rival group of arcane spellcasters, but scant evidence backs any such conclusion. Others suggest arcane oozes arose as a natural response to some cataclysmic event caused by arcane magic. Adherents of these later theories claim that one day arcane oozes will drown out all arcane magic in the world.