Spell OLD

Stage 1: Magstick (Magic + Stick, what the wand is made of, = Magstick). Type: Psychic. Dex Entry: "The only possible explanation scientists have came up for it is that the tree that birthed it must have developed emotions. The psychic entity relies solely on the stick for existence."

Evolution Option 1: Cauldevil (Cauldron + Devil, due to your description, = Cauldevil). Type: Poison/Psychic. Dex Entry: "Upon evolution, the stick and psychic entity have liquefied and fused into a form of sentient, smoky goop. The cauldron somewhat resembles a gnarly tree knot."

Evolution Option 2: Spbook (Play on the words Book, Boo and Spook, = Spbook). Type: Fairy/Psychic. Dex Entry: "Unlike many trees, the once tiny floating stick has found a very quick way of going from bark to book. The printing agency has been conducting research on whether or not this Pokemon could unlock the secret to quick paper-making. It is known to play tricks in libraries. Due to this pranking, people thought it was a spooky ghost before it was confirmed to be entirely different!"

Feel free to use! 

Also, if I may, I have alternative evolution ideas, if you'd like to use them. The cauldron stage could be achieved be leveling up with a metal coat whilst knowing the move "Smog" or "Acid Armour" as you suggested. The second could evolve whilst holding the item "Genius Wing" and knowing the move "Aromatic Mist" or "Crafty Shield". I suggest the items because they give their typing more reasoning - the metal coat would make sense for the cauldron, literally coating the stick in metal, making it cauldron-like. The genius wing could be seen as a quill, which medieval book writers would use to, you guessed it, write with, therefore giving the book good reasoning too - also it's quite medieval-like in concept so that's a bonus. Hope you like the ideas.