It contains the collected and condensed negative psychic energies of Nemesis Prime's victims; their pain, agony, suffering and despair. The minions of Unicron put the Doomstone into Ravenus and used his enhanced Powerlinx command to kidnap the "free" Mini-Cons Twirl and Servo, planning to use the link formed with the Powerlinx to release the Doomstone's energies into the Linkage, destroying the souls and minds of all Mini-Cons and returning them to the mindless engines of destruction they were created to be. Should the crystal be improperly handled, its energies would wipe out all life on whatever planet it was on—in this case, Earth.
The minions of Unicron were just a hair too late, though. Thanks to the intervention of Redline and Kingbolt, Ravenus was contained, and the newly-enhanced Mini-Cons were able to extract the Doomstone. Redline sealed it inside his extra-dimensional storage space to protect it from harm until they could figure out how to deal with it properly.