Discord is wanting to settle down with Fluttershy and stay with her full time, but as the god of Chaos he'll cease to be if he acts to normal and it becomes clear that no normal creature can stand chaos 24/7 and he seeks a replacement, he ends up landing on Spike for the job.
Spike is missing all the time he and Twilight use to spend together, even if it was mostly work, and ends up using his powers so he can earn her respect as an equal instead of being seen as a lowly assistant.
While planning his first move he ends up enlisting the Cutie Mark Crusaders aid so none of the other mane 6 feel left out of the fun.
Featured (Apparently) - 8,28,21 - 11,19,22 - 02,19,23
- This Story takes place after Season 8 but before Season 9 -

After another defeat at the hooves of Celestia, Chrysalis comes across two dead dragons and a cracked egg. She takes the egg and hatches it believing it could be the weapon she needs against the ponies of Equestria. However, as the dragon grows, learning from his brothers Thorax and Pharynx and with the attack on Canterlot approaching, Spike will leave home to discover for himself whether he will follow his mother's orders or try peace as his brother argues for.

This story is a sequel to Spike: Lord of Disharmony and Chaos

Discord retired, yes, he retired from being the lord of chaos; and now Spike is his replacement.
With fresh new ideas and all the energy of youth, Spike is ready to make the life of the Mane 6 impossible; along with his loyal partners: the CMC

As all my other works, this is a translation. You can find the original in spanish here

The ponies of Equestria and the dragons of The Dragon Lands are waging war with one another. The battles are long and brutal with the Equestrians being on the losing side. When the officials gathered together, they figured out a possible solution to their problem. They lacked knowledge of the dragons and seeing as to how no dragon was willing to tell them how they worked, Equestria decided to take the knowledge by force. Join Test Subject SP-1K3, formally known as Test Subject D-1043, and other dragon test subjects as they try to escape their lives as lab rats.

Moving this to the top, because apparently, people can't read all the way down:

To Whom It May concern, this is canon divergent, and the circumstances in this fic are but the best delivery for the situation that I wish to create. It is not an accusation fic, and those elements will be nothing but background to the primary events of the story. if you added it to one of those collections, please remove it.

He groaned as he woke, and he opened his eyes to the same dirt road he’d fallen asleep aside. Maybe it was the crying, or just the situation hitting him in full force, but Spike woke up exhausted, not even wanting to move an inch from his sleeping position under the tree. He layed in the grass, eyes half lidded, but awake, and he stared at the ground next to him, watching a band of ants trailing along to their anthill. He flinched as he felt a ladybug of all things land on his nose. He stared at it angrily, and given the reason he was out here, he felt it was well deserved. A twist of good birthday luck spurred on an instinct that he didn’t even know he had and a power that he didn’t even know he had turned his happy birthday and the next few days into the worst bad luck he’d ever experienced.

And if this ladybug was meant to represent good luck like ponies said it did, he really could’ve used it two days ago.

Still, his anger crumbled to pieces at a thought that he’d shied away from, because he wanted to believe more than anything that it wasn’t true, he had every reason to think it wasn’t true, but still, it cropped up, like an annoyingly persistent weed that the farmer of his field of thoughts had to tend to and yank up every few minutes.

Even if he hadn’t known this would happen, he could’ve stopped it. He could’ve stopped taking things, stopped trying to build a hoard and now what? He didn’t and he was paying the price for greed. What was the word Twilight used? Avarice. It was Avarice he’d had, and now he was paying the price.

It was all his fault, and he was stuck with the results.

Ponykind knows so little about dragons, and the Greed Growth incident came unexpectedly because of that. Spike, having the terrible magnitude of incident come crashing down on him, runs away from Ponyville, along the way meeting an eccentric Satyr, and encountering the Changelings, as well as whole cabal of powerful deities. Hoping to improve the condition of the Changeling Hive, and wanting a reason to stay away from the Ponies who know so little, and have so much to fear, he finds himself psuedo-adopted by the Changeling Queen. With new powers, a whole host of issues and conflicts to resolve, and friends to try and help from a distance, Spike's hands are more than full, especially with trying to stop Queen Chrysalis from starting a war.

The static whispered to him. It spoke of power, indefinite and unbeatable. He’d stand over the likes of Discord, Celestia, and Luna. Twilight and Cadence would be nothing in comparison. They spoke of greatness, destruction, and creation.

They spoke of sea and sky.

They spoke of earth and stars.

Of demons and gods.

Phoenixes and dragons, snakes and birds.

He dreamt of rocks and water.

He woke up.

He didn’t remember his dream. The dream.

A dream of rocks and water.

Ten years ago: the coronation of Princess Twilight Sparkle, the goddess of knowledge and power occurred. One year later, Spike the Dragon disappeared. Twilight Sparkle was broken. With the help of her fellow princess and bearers, as well as all of Equestria, she seems to have accepted the loss of her best friend.

On the nine year anniversary though, she, three other Bearers and Luna make a trip to the Western Badlands. Their mission? To get to Blackwater City, and arrive at an international political convention. However, things are turned upside down when they find out that their escort across the Badlands is the "Dragon Outlaw", Spike. However, he is not the same innocent child as before. Even though he is still biologically a child, the harsh world of the Badlands has forced him to grow up fast. Can Twilight help bring him back to her, or have the Badlands claimed his life as it has so many others? After all, once an outlaw, always an outlaw.

(Inspired by this video from Scribbler and this Deviantart comic from Epulson)

Spike sets off on a solo camping trip as a vacation given to him by Twilight Sparkle for being such a hard worker, when he sees an unexpected face.

Maud Pie, Pinkie’s more serious and stony-faced (pun intended) sister, shows up at his house bearing gifts for him for the trip.

Spike knows Maud well enough that she’s trying to be nice, but why? Why is she? She hardly even knows him.

Or does she...?

Spike is killed in a tragic train crash while taking a weeks vacation to Manehatten without Twilight by his side. Twilights grief goes beyond what the others ever expected, and they band together to try and help her through it. However, things turn more serious when she begins a strange experiment that put both herself, and her close friends and family, in grave danger.

When Twilight and her friends started asking their draconic friend, Spike, about what he's done in the Dragon Lands after the following years of Twilight's coronation, they are in for a tale.

End of Season 9
Please note that I think this what happened before Spike returned to Equestria to help Twilight.