Spark Regenerator Key

When Optimus Prime is your leader, it pays to take out insurance.

The Spark Regenerator Key is an Autobot artifact that can restore life to a dead Spark, effectively resurrecting a deceased Transformer. Taking that into account, it's surprisingly small.

Fiction

EarthSpark cartoon

When Prowl set off for Earth via spaceship, as the space bridge was inoperative, he wasn't sure what state he would find the Autobots in and therefore took the Spark Regenerator Key with him. His ship crashlanded on arrival, being out of energon, and he was concerned the artifact would fall into Decepticon hands. Optimus Prime was surprised he'd brought it with him. Mo, recalling her mother's advice to never leave the house without your keys, took the artifact with her, attaching it to a string necklace. When the Decepticons later stole Prowl's ship, she revealed to the others that she'd kept the artifact safe. No Soldier Left Behind

Evidently the Autobots let Mo continue to carry the key. As they were under attack from the Quintessons and needed a way to destroy the Quintesson ship, Mo recalled she had the key and had Nightshade take her to Terratronus's spark chamber. There, she held the key up and it was sucked into Terratronus's spark, resurrecting the Titan. Judgement Day, Part 2