Dehacked information and tutorials
What is DEHACKED?
Dehacked is a fairly old DOS program (that will run under win95/98) made to allow you to hack your way into the doom and doom2 EXE files and modify the way they work. It was programmed by Greg Lewis. It can allow you to change how the various things in the game (monsters, decorations such as pillars etc) behave and sound in game. You can also modify the way weapons work, the text messages displayed in the game and a whole host of other things too.
What type of things can it do?
You can make inanimate objects move and attack. You can give monsters different attacks and sounds. You can make scenery things look different and behave differently. You can change a things sounds. You can make weapons more/less powerful or quicker/slower or behave quite differently from the doom originals. You can make things explode when they die. You can make one kind of monster look like another, or even a player. You can change on screen messages and there are a whole host of other tricks you can try.
Note, dehacked changes the way the doom executable file works. It does not create levels, but the things in levels, either created by Id or someone else, will behave differently because of dehacked.
What is BEX?
When team TNT released their Doom source port "Boom", they extended much of the flexibility of the Doom engine. One of the features they added was the ability to load dehacked data files (or patches) direct from the command line. Previously you had to use dehacked to create a new EXE using the data in a patch and then use that EXE to run doom.
Distributing modified EXE files went against Id softwares instructions but distributing data patches was accepted. The file size of a data patch was much smaller too. In addition to making it easier to load a dehacked patch, team TNT also added a whole host of new commands and options to the patch format. These Boom extensions gave rise to the BEX name. To add BEX information, you need to edit the patch file manually with a text editor. At the time of writing, there is no BEX specific editing program.
A number of source ports also have the ability to load dehacked patches from the command line and some also support the Boom extensions. Zdoom is one such port.