What happens when you beat black tower in Pokemon 2?

There's no "Black Tower" in Pokemon 2 (Gold and Silver). You're likely thinking of the "Silph Co. Building" which is a major location in Pokemon Red, Blue, and Yellow.

Here's what happens when you complete the Silph Co. Building in those games:

* You rescue the kidnapped Pokemon: The leader of Team Rocket, Giovanni, has kidnapped Pokemon and is using them for his own evil plans. You need to fight your way through the building to save them.

* You defeat Giovanni: You'll eventually face Giovanni in a challenging battle. If you win, you'll liberate the building and get a special item (Silph Scope), which helps you deal with Ghost-type Pokémon.

* You contribute to the story: Completing the Silph Co. Building is a key event that helps you progress the story and uncover more of Team Rocket's plans.

In Pokemon Gold and Silver, the "Silph Co. Building" is absent. There are completely different locations and a different story arc with a new villainous team, Team Rocket.