In a secret area below the surface of the Valley of Living Rock lived eight unexpected inhabitants. They were not human, but they also weren't rock troll either. They were a completely distinct species: Cybertronian. All of them were Autobots, and they had arrived five years ago and mysteriously shrank to a tenth their original sizes when they did. They eventually encountered the trolls and learned that they had shrank because of an old security measure they put in place to protect themselves from falling projectiles. When the leader, Grand Pabbie, learned that they meant no harm, he attempted to reverse the effects, but the trolls didn't think ahead when they put the spell in place and made it permanent all those years ago, and thus the Autobots were stuck being extra small.
Optimus Prime was the leader, who now stood at three feet, two inches tall. With the war for Cybertron and the Decepticons things of the past, he struggled with trying to find things for him and his team to do. Grand Pabbie felt that the humans were not ready to behold them yet, so he created an area below the surface for them to reside. They recreated some Cybertronian technology in their basement-like area and mainly used it to monitor the land should trouble come. Though they weren't as big as they once were, they were still Cybertronian: tough to destroy and not about to run from danger.
Ultra Magnus was Optimus's blue second in command, and was just a single inch shy of three feet. Despite promising to keep an open mind to any and all humans they might encounter, and making the other 'bots agree and promise to do the same, Grand Pabbie still didn't think the humans were ready to behold them. He started growing a little restless, seriously thinking that disciplining the Wreckers back on Cybertron was a better situation than the one they were now in.
Bulkhead was their biggest non-commander fighter, even if now he was only 2 foot 7. Not getting to leave the basement much made him vent all his energy whenever a chance came, destroying so many random rocks that the trolls marked a large area just for him to keep the damage he caused from spreading to the point where visiting humans would not accept rock slides as reason behind it.
Ratchet was their orange and white, two and a half foot medic. Although he enjoyed the peace and quiet, having proved his metal during the war more times than could be counted, he did find himself starting to get bored with the lack of activity. Trying to recreate a ground bridge could only keep him busy for so long.
Smokescreen only barely got into the action of the war before the 'cons were defeated and their home planet became uninhabitable. Being reduced to 2 foot 5 did nothing to drain his desire for the action and adventure his teammates got so much more of than he did. Permission to go up to a mile away from the valley whenever Grand Pabbie said it was safe was not enough, he wanted more.
Wheeljack was glad that he was with the 'bots he was with, and found that just because he's only 2 foot 3 doesn't mean he can't still show off his Wrecker skills. Just like everyone else, though, the longer he didn't have anything to do, the more restless he became. He had re-created so many grenades in his downtime that Grand Pabbie and a few other trolls had to carve out a storage room for them all. At this point, it was almost full.
Bumblebee lost his voice and was only 2 foot 1 now, but the 'bots and the trolls could understand his rhythmic beeping just fine and he was surprisingly the least restless of all eight 'bots. Should activity that warrants their presence come, he'd be fine with a chance for more action as long as he doesn't lose any more parts of himself. Literally.
Arcee wasn't that big by Cybertronian standards to begin with, having been 15 feet tall next to her 20 foot and taller comrades, but being reduced to 1 foot 6 while everyone else was at least two feet or taller did grind her gears a little. Losing two partners to the Decepticons hurt her mentally a lot, but Bulkhead and Wheeljack, who both lost many Wrecker friends to the strongest 'cons, helped her get through and survive the final battle.
After two years of very little happening outside of their occasional trips outside, when Ratchet's re-created technology detected supernaturally caused abnormal weather conditions, they were eager to help stop it, but Grand Pabbie needed to inform them of what they would be up against first. When the troll's human friend Kristoff arrived on his trusty reindeer Sven with a girl and a sentient snowman in tow and requested Pabbie's presence, one troll said he was napping when in reality, he was in the middle of telling them everything they needed to know. The moment he detected the ice in Anna's heart, he went up to check on her, but ordered the eight Autobots to stay below, much to their disappointment. Their disappointment only got worse when the problem was solved about a day later.
Three more years passed, and with Elsa's powers under control and Arendelle having long achieved peace with most other countries, the Autobots were more bored and restless than ever. Even Bumblebee was starting to get frustrated by the lack of activity. They heard the other trolls talk about Anna and Elsa, their parents, the story of their parents escaping the enchanted forest, countless times. And all it did was make their desire to meet them grow.
Then one night, Ratchet's monitors went off, detecting supernatural activity in Arendelle.
"Optimus! Arendelle is getting attacked by unidentifiable supernatural forces!" He said.
The seven other Autobots went to the monitors as an alarm went off throughout the basement.
"It does not seem to be caused by Elsa's powers this time." Said Optimus.
"So what could it be?" Asked Wheeljack.
"Whatever it is, it's nothing good." Said Bulkhead.
Suddenly, Grand Pabbie entered the room through the elevator hidden by black rock.
"Grand Pabbie, we're ready this time." Said Smokescreen.
"We know about the sisters and the guys they trust." Said Arcee.
Bumblebee beeped, which the others understood as: "We've been waiting for a chance like this for five years! Come on!"
"With your permission, sir, I think it's time we meet them." Said Ultra Magnus.
"Very well. Fire up your portal generator. Me and the trolls will meet you there." Said Grand Pabbie.
Ratchet entered coordinates into one of his monitors and pulled a set of twin levers beneath it. A swirling blue and white portal opened up.
"Autobots, it's time to see Arendelle!" Proclaimed Optimus.
All eight of them transformed into vehicle mode and drove into the ground bridge portal at full speed. At long last, another chance for the action and adventure they so desperately crave had arrived. And this time, they were taking it.