This attachment script from Greg Scherrer allows you to set up soft-fade dimming scenes with just a few simple AppleScript commands. Using the methods in the attachment script, you can easily start dimming, stop dimming, control how fast the dimming should occur, and optionally execute an action group upon completion.
After downloading, drag the _dimming system attachment.scpt_ file into:
_/Library/Application Support/Perceptive Automation/Indigo 2/Scripts/Attachments/_
Next, follow the configuration instructions outlined in the comments at the top of the script file.
Examples and instructions are provided in comments at the top of the script file and in this forum post.