Sonic Frontiers' story does an excellent job of tying up most of its loose ends and resolving the initial mysteries that drive it forward, but players who got the normal ending will have noted it ends on a rather bittersweet note for a certain character. Spoilers beyond this point.
Sonic Frontiers introduces Sage, an artificial intelligence created by Doctor Eggman which has grown far beyond his initial expectations. Eggman sees Sage as a daughter, and Sage spends much of the game acting as an antagonist toward Sonic and friends due to wanting to keep her Father safe. Her growth over the course of the story ultimately leads to her joining forces with Sonic to face off with The End, a terrifying spacefaring monster that devours worlds, and Sage seemingly dies dealing the finishing blow. Her status after the fight is left ambiguous but the normal ending strongly implies her death.
How To Get The True Ending
In order to get the true ending players will need to play the game in hard mode. On Normal difficulty, and below, the true final boss cannot be fought, and the true ending is impossible to obtain. Luckily, players don't need to complete the entire game on hard mode to achieve this ending, as long as the difficulty is set to hard before the sixth Chaos Emerald is acquired on the last island.
Players will then need to defeat the Titan "Supreme", whose array of ranged attacks and bullet hell-esque attack patterns make for a difficult encounter. This is all standard for finishing the game as normal, however, and the true challenge of this ending comes once Supreme is beaten.
Once Supreme is beaten Sonic and Sage set off into space to face off with the true form of the ancient evil that the Ancients faced years ago, and come face to face with the malevolent moon known as "The End." This is where the gameplay style shifts to a top-down bullet-hell shooter reminiscent of Ikaruga, a game originally released for the Sega NAOMI arcade platform. Players will be familiar with this style of gameplay from the hacking sequences earlier in the game, but the difficulty is much higher and The End can fire off powerful lasers that neither bullet type can counter.
If players successfully break through the brutally difficult fight with The End and then succeed at the Quick-Time Event afterward, Sonic and Sage will put an end to The End, with Sage seeming to sacrifice herself in the process.
What You Get For Beating The True Ending
Achieving the True Ending allows players to view an additional scene after the credits have rolled, revealing that Doctor Eggman has managed to rescue Sage and restore her inside his computer systems.
Sonic Frontiers is available for PC, PS4, PS5, Xbox One, Xbox Series X/S, and Nintendo Switch.