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"