So my party was traveling through a mountainous region, and I rolled for a random event. The party was traveling, and then, they found a sleeping purple worm. It was mostly underground but its head was exposed. Every party member wanted to do something different. The fighter wanted to get the jump on it and kill it. the ranger wanted to make friends with it. but the Druid had a very different idea. She got her components together and casted awaken. this took a while what with the 8 HOUR casting time, but it was done. the worm woke up and felt like a whole new monstrosity! he got up and simply asked, "Where am I?" the party explained everything to the worm. He asked what his name was. the druid said "maybe......... Vagor?" The worm was totally into his name. he responded with, "Oh yeah, that's a good name!" his alignment was Chaotic good. As thanks for showing him there's more to life than mauling people to death, Vagor escorted them out of the mountains. Okay two things. 1: you can't cast awaken on gargantuan monsters. 2: you can only cast awaken on beasts or plants. the Purple worm is a monstrosity. But I figured it was such a cool idea and could make such a cool character, I broke the rules for this. What do you think about the story? Post opinions below! Cheers! 

If the DM allows it, it is fine. Personally, I think that is a humorous story.

Thanks!

I love it and I think it’s awesome!

Great! Glad you like it!

As the DM, you are the final authority on the rules. If you say it works in your game, it doesn't matter what published stuff says (I would add, as long as your understand any balance and fairness implications for changing things)

It was awesome, outside of the box thinking, sure why not allow!

The Rule of Cool

Basically, as a DM you can authorise one-time-only moments where the normal restraints and rules are bent or broken in order to provide a cool and memorable moment. Just be sure to let the group know this is a special one-off. 

If you are changing the rules on a more ongoing basis (the Awaken spell can now be cast upon Monstrosities) then ensure consistency: anyone with the spell can do this, and this might result in the group encountering Awakened monstrosities in future. Be sure to let the group know this, especially if more than one has access to that spell and be sure there's still balance: if you went with this, then you've given your Druid a buff and given the rest of the party nothing. Be sure to talk to the group about such rule changes to ensure there's no resentment.