Like in prior Sonic games, there is indeed a Sonic Frontiers secret ending that you can unlock depending on how you play the game. Luckily, it won’t change up the ending cutscenes all that much and will only impact the final boss fight. And yes, there’s a true final boss too.
How to get the Sonic Frontiers secret ending
To unlock the Sonic Frontiers’ secret ending, you need to play the game on the Hard difficulty setting.
That said, you don’t have to switch it to Hard mode until right before Sonic claims the sixth Chaos Emerald on Ouranos Island, the final island in the game. So you can freely play the game on Easy or Normal before that point, raise the difficulty near the end, and get the secret ending with no issue.
Going into the last boss fight on any difficulty other than Hard will pit you against Titan Supreme, which is really just a series of quick-time events. But if you do go into it on Hard mode, you will face The End instead in a final shoot-’em-up battle.
Sega fans will recognize this last battle as a reference to the notoriously difficult Ikaruga shmup, but luckily this fight is not as challenging as that.
For more guides on Sonic Frontiers, this is how to increase Sonic’s running speed and whether the game is coming to Xbox Game Pass.