The closest thing to a "dragon-spell" would be the "spell" cast by Smaug the dragon on his treasure hoard:
* "All is mine! All is mine!" This is the dragon's fierce and powerful claim on his wealth, which holds a sense of enchantment, making it difficult for anyone to take anything from him without consequence.
While not a literal spell, this constant assertion of ownership by Smaug acts as a powerful force that protects his hoard and strikes fear into the hearts of anyone who dares to approach it.
It's important to remember that Tolkien's world is not a traditional fantasy setting with magic as a primary force. While magic exists, it is often subtle and woven into the fabric of the world, rather than overt and spell-based.