Caged Eternity

Gaseous Form. the caged eternity can move through spaces as small as 1 Inch across and may end its turn in a hostile creature's space. Whenever a creature ends its turn within a space occupied by the caged eternity that creature takes 7 (2d6) psychic damage. When killed the creature dissipates into a harmless mist.

Crushing Presence. the caged eternity has advantage on attack rolls against creatures within its space

Magic Resistance. The caged eternity has advantage on saving throws against magic.

Spellcasting. The caged eternity is a 17th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The caged eternity however, has no spells known or prepared instead relying  on spells gained though the creature's Spell Reverberation Feature.

Spell Reverberation. Whenever the caged eternity is hit with a spell that spell is added to the creatures spell list as a spell of the same level as the one that was cast.

Eternal Torment. Whenever the caged eternity is killed it returns to life at full HP 1d4 days later.

Multiattack. The caged eternity uses it's Mind Fog then makes two slam attacks.

Mind Fog. Each creature in the aberrations space must make a DC 20 Intelligence saving throw or become stunned until the aberration's next turn. A creature can repeat the saving throw if it takes damage or another creature uses an action to wake them

Slam. Melee Weapon Attack: +9 to hit, reach 0ft., 1 target. Hit: 19 (4d6 + 5) Bludgeoning damage.

Counterspell. The caged eternity chooses and prevents one spell as if by the Counterspell spell.

The aberration can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The aberration regains spent legendary actions at the start of its turn.

Slam Attack. The aberration makes a slam attack.

Replicate Spell (Costs 2 Actions). The caged eternity casts a spell stored with its spell reverberation feature without expending a spell slot. that spell is then removed from the creature's spell list