The Post-Scratch Trolls about to receive the Ultimate Reward
The Ultimate Reward is what awaits players who have successfully won a session of Sburb. After the necessary actions for winning have been performed, a victory platform appears near Skaia. This platform features a large version of the Sburb logo with a door on it; the door serves as an entrance into the new universe the players have created.
Karkat reaching for the Ultimate Reward
What, if anything, the Reward includes beyond access to the new universe is currently unknown, as none of the five sessions shown in Homestuck have managed to successfully claim it. The A2 trolls came closest, as they were able to achieve victory, but were prevented from getting their Reward when the B1 Jack Noir arrived in their Incipisphere and destroyed the door before they could open it. The A1 trolls and B1 kids were unable to win and had to scratch their sessions. The B2 kids are stuck in a void session, but it will become winnable when the B1 kids arrive and deposit their Battlefield and planets into the Incipisphere. Caliborn's dead session is unable to produce a new universe, so its victory and defeat conditions (and presumably its Ultimate Reward) differ from that of a normal session.