The AllSpark is an ancient source of energon and power. It is pure energy and the source of all life on Cybertron.
The AllSpark was the very heart of Primus himself. During the Great War, Optimus devised a plan to move it away from Megatron. Alpha Trion forged an invincible container to hold its immense power. It was sent to Theta Scorpia as the sector was hazardous to anything that lived.
When the war ended, Optimus and Wheeljack retrieved it. While they were on route back to Cybertron, they learned that Unicron had returned. Unicron went for the AllSpark to destroy it when they returned, but was blocked by Optimus. In fear of Unicron devouring the Allspark, Optimus released its energy into the Matrix of Leadership, making it both the Allspark and the former at the same time.
Optimus flew away from Unicron to reach the Well of Allsparks, but was shot down by Unicron and had the "Allspark" snatched it from him. Unicron opened the vessel that once held the Allspark and, just before realizing it was a trick, Unicron's Anti-spark was sucked into the vessel, tearing his pure energy form from Megatron's body, trapping him forever.
Optimus told his team that he must go into the Core in order for life on Cybertron to come. Bumblebee promised to Optimus that they will keep the peace in his name. Optimus smiled at them and flew into the Core. Optimus became one with Primus and millions of Sparks fly into the sky. One red spark, possibly Optimus' spark, shines out.
2015 Robots in Disguise cartoon
Upon his return to Earth, the evil Megatronus tried to execute his doomsday plot of using his spark fuser to combine the Anti-Spark of Unicron from Earth's core with the AllSpark from Cybertron in order to destroy both planets. He activated his staff, causing a portal to open in the sky and the AllSpark to come through, but Optimus managed to shatter the fuser, sending both AllSpark and Anti-Spark back to their resting places and ruining Megatronus's plan.
- Normally Ratchet is the one who mentions the AllSpark, as he says, "By the AllSpark!"
- Wheeljack hoped the AllSpark was bigger than he had imagine. Looks like a Transformer live action movie reference.
- Usually when someone says that someone has become "one with the AllSpark", they mean they have died and is in a heaven like universe.