Scenarios can be created using the scenario editors of Blades of Exile or Blades of Avernum. Levels, characters and graphics are fully customizable. Significant differences exist between Blades of Exile and Blades of Avernum, not the least of which is the significantly more capable (and complex) scripting language built into Blades of Avernum.
It is worth noting that the source code to Blades of Exile has been released under the GPL, effectively making it free to anyone willing to download it.
The Encyclopedia is designed with prospective scenario designers in mind.