Create a map with a circle mask that shrinks

Hi,
I’m newbie to unity but i’m learning fast.
What i want to achieve:
A map(terrain) which is a circle/square (doesn’t matter). I want an outside circle/square of this map which will basically come towards the center of the map and damages characters :like a deadzone, a gas zone (see Warzone).

My question is how can i achieve this? With a map that shrinks or with a mask that becomes bigger? The final product will be a mechanic how Warcraft 3 - Warlock map has.

Thanks

You can do this in many different ways,

You can scale down the map Transform.
You can use a sprite mask and decrease the borders.
You can have different map gameobjects that are enabled/disabled.

The way you do it is the way it is move convenient and better optimized for your project. For example, Warcraft 3 Warlock map is not updated in real time, it uses something similar to a Coroutine and changes the map every X seconds.

However if you are planning to do something in real time, you can use the Update method scaling down the map transform and having an outside collider that damages the player.