In order to play the mission first download it then extract the contents.
If you have the Deus Ex SDK installed:
Open UnrealEd for Deus Ex, open the map named 16_Intro.dx and hit CTRL-P
to play it.
If you don't have the Deus Ex SDK installed:
1. Download the multiplayer patch from www.deusex.com.
If you have Deus Ex Game of the Year Edition this should not be necessary.
2. Go into Deus Ex and view the keyboard/mouse controls, at the bottom
of the list will be a button bound to "Send Message to All"
bind this to a key you can remember like T.
3. On the title screen press T (the button you bound), back space to remove
"Say", and in its place type:
set JCDentonMale bCheatsEnabled
No confirmation message will appear but if you did it right cheats will
then be enabled.
4. Press T again, delete "Say" and type : legend
A secret menu will appear with an option to load a map. Click that button
and select : 16_Intro.dx from the list to start the mission playing.