Potion of Planar Infusion

Potion, very rare
  Meticulously extracted from the touch of starlight, the sound of a vacuum, and the the light from a black hole, this silver liquid forms a swirling, spiral galaxy when left to settle.
  Planar Infusion. After imbibing this potion, your mind, body, and soul becomes bound with that of an entity from another plane. Roll a d8 and consult the table to determine the entity. You gain the statistics of that creature as per the polymorph spell. These effects last until 10 minutes elapse, you fail a saving throw against the banishment spell (which banishes the planar entity, not you), or your assumed form is reduced to 0 hit points. Dispel magic does not end the potion’s effects.
  d8 - Plane Creature
  1 - Air Air elemental
  2 - Earth Earth elemental
  3 - Fire Fire elemental
  4 - Water Water elemental
  5 - Fey Green hag (with spellcasting)
  6 - Shadow Wraith
  7 - Positive Unicorn
  8 - Negative Incubus/Succubus
  Consequences. After the duration elapses, your mind is strained and can even break. Make a DC 15 Intelligence saving throw. On a failure, you take 6d6 psychic damage and are insane until you finish a long rest. While insane, you can’t take actions, can’t understand what other creatures say, can’t read, and speak only in gibberish. A greater restoration spell cast on you ends this effect.