Smply proceed forward, down the tower into Yggdrasil’s core, defeating all the guardians. As usual, once you see the entrance to DigiLan, save and heal up, because the final boss fight is right after!
And that’s the end of the game! Here’s a list of things you can do post game:
-Complete the Royal Knights quest
-Complete 7 Demon Lord Quest
-Complete Offline Colossuem
-Complete your DigiDex