Dreams can take strange turns, but lucid dreams are not supposed to. That’s the whole point, to change the dreamworld through lucidity. However, when he tried messing around with his own subconscious, William signed up for far more than he ever thought he was going to.
Now, Reality seems to be following arbitrary rules, cartoon logic and the fact that he cannot change back from his ridiculous alicorn form is just vanilla icing on the bizarre cake.
And unbeknownst to him, a certain Princess is infuriated by the intrusion in her domain.
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That awesome piece of coverart was drawn by the equally awesome mix-up, that you can also find on his dA account here.