Terraria End Game

There is no single official end goal. However, there are a few long-term goals the game sets:

• Fight all boss monsters (kind of the minimum thing you have to do before you can claim to have beaten the game)
• Get/craft all equipment (requires fighting many bosses multiple times).
• Completely eliminate all corruption from the world (the Dryad wants you to do this, but it would be an incredible amount of repetitive work).
• Collect all achievements.

And of course you can do all this in mediumcore or hardcore for an extra challenge.

Apart from that, many players make up goals for themselves, often in the form of building elaborate structures.