I want to fade the player when he goes out of the area.

For example, suppose a person enters a building. When a person is outside of the building, he shouldn't be seen, but when he enters he gradually becomes visible (using fading effect).

when elephant is inside green area his alpha should be 1

when elephant is outside of green area his alpha should be 0

You can use world space to fade your character.

The object space (or object coordinate system) is specific to each
game object; however, all game objects are transformed into one common
coordinate system — the world space.

If a game object is put directly into the world space, the
object-to-world transformation is specified by the Transform component
of the game object.

https://en.wikibooks.org/wiki/Cg_Programming/Unity/Shading_in_World_Space

Fading by world space

Shader "Smkgames/worldSpaceFade" {

      Properties{

         _Size("Size",Vector) = (2,2,0,0)

      }

   SubShader {

      Pass {

         CGPROGRAM



         #pragma vertex vert  

         #pragma fragment frag 





         struct vertexInput {

            float4 vertex : POSITION;

         };

         struct vertexOutput {

            float4 pos : SV_POSITION;

            float4 position_in_world_space : TEXCOORD0;

         };



         vertexOutput vert(vertexInput input) 

         {

            vertexOutput output; 



            output.pos =  UnityObjectToClipPos(input.vertex);

            output.position_in_world_space = 

               mul(unity_ObjectToWorld, input.vertex);



            return output;

         }

         float2 _Size;



         float4 frag(vertexOutput input) : COLOR 

         {

             float3 world = input.position_in_world_space;



          float4 equation = pow(world.x/_Size.x,8) + pow(world.z/_Size.y,8);

          return smoothstep(1,0,equation);

         }



         ENDCG  

      }

   }

}

Surface Shader

After understanding world space we can use it in our surface shader:

Shader "Smkgames/worldSpaceFade" {

    Properties {

         _Size("Size",Vector) = (2,2,0,0)

        _Color ("Color", Color) = (1,1,1,1)

        _MainTex ("Albedo (RGB)", 2D) = "white" {}

        _Glossiness ("Smoothness", Range(0,1)) = 0.5

        _Metallic ("Metallic", Range(0,1)) = 0.0

    }

    SubShader {

 Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}

     Pass {

        ZWrite On

        ColorMask 0

    }

        LOD 200



        CGPROGRAM

        #pragma surface surf Standard fullforwardshadows alpha:fade



        #pragma target 3.0



        sampler2D _MainTex;



        struct Input {

            float2 uv_MainTex;

            float3 worldPos: TEXCOORD2;

        };



        half _Glossiness;

        half _Metallic;

        fixed4 _Color;

        float2 _Size;



        void vert (inout appdata_full v, out Input o){

            UNITY_INITIALIZE_OUTPUT(Input,o);

            o.worldPos = mul(unity_ObjectToWorld, v.vertex);

        }





        UNITY_INSTANCING_BUFFER_START(Props)

        UNITY_INSTANCING_BUFFER_END(Props)



        void surf (Input IN, inout SurfaceOutputStandard o) {

            fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;

            float4 equation = pow(IN.worldPos.x/_Size.x,8) + pow(IN.worldPos.z/_Size.y,8);

            o.Albedo = c.rgb;

            o.Metallic = _Metallic;

            o.Smoothness = _Glossiness;

            o.Alpha = smoothstep(1,0,equation);

        }

        ENDCG

    }

    FallBack "Diffuse"

}