What are 404 pages ? 

its a page or file which is no longer available on website,

Advantages of 404 pages in website Defacing :

if someone will trying to acess to your shell by guessing like (c99.php, r57.php etc) then it will show your custom page on Not found Link.

in Defaced site usually we upload our deface page as index.html or anyname.html

and we can see our deface page on That custom link only where we uploaded our deface page 

like : site.com/index.html, site.com/r00t.html site.com/r00t.php

by adding deface page's code in 404 page you can see your deface page on every link 

like site.com/xyz.php site.com/x.html site.com/xyz/, 

                                                   Before

 

                                                     After

How to do it ?

1-open Note Pad

2- paste your deface page's code in noatepad

3- save file as 404.shtml 

5- now upload this file in public_html/ directory using shell

now check any error link !!

Code for 404 Pages :

<style type="text/css">body { background-color: #000;}</style>
<img src="http://a3.sphotos.ak.fbcdn.net/hphotos-ak-snc7/s720x720/432146_157462177702828_100003171391108_222151_895998112_n.jpg" width="1198" height="765" />
(replace image link with your own image in this code)