Spark of Combination

The Spark of Combination is a gift given to the Autobots by Primus at the beginning of the Energon arc of the Unicron Trilogy. Given to Optimus Prime, the Autobots were granted the ability to combine themselves into larger, more powerful forms. Essencially, everybody became a combiner and Optimus, Jetfire, and Overload aren't special anymore. The Spark of Combination is kept in a Matrix-like device in Prime's chest, which is visible when combined with his team of mindless, soulless worker drones.

From Teletraan I: The Transformers Wiki, a Wikia wiki.