Alright, here is how this works:
1. Start off with any adventurer you please.
2. Have a wight come over and kill them with Negative levels. Let the adventurer rise as a Wight.
3. Kill the 1st Wight. The adventurer starts taking levels in Emancipated Spawn.
4. Everyone go YAY! Cast any spell that changes type on them. Find a Spectre; have the Spectre turn the non-Undead Wight into Spawn.
5. Kill the Spectre. The adventurer again takes 3 levels in Emancipated Spawn. He now has all the special qualities and special attacks (thanks to how the 3rd level of Emancipated Spawn works) of a Wight, a Spectre, and his original form.
...
N. After repeating this process a bajillion times with different spawn-making Undead, you literally have all the abilities of all of the Spawn-creating undead that have ever been published. This includes such gems as immunity to positive energy effects (a Visage gets this; it doesn't include Turning, but since you get a boost to Turn Resistance of at LEAST +6 for every time you have gone through this cycle...), a negative level inflicting song, and a bunch of spell-like abilities. Plus, since you can level a bit every time you become a Spawn, you could go through a cycle of:
Low HD spawning monster/Class levels X/Emancipated Spawn 3
To get most of the class features in the game.
Pick a final Undead with good HD and ability scores (eh, it might be nice if the starting character had a bunch of their stat increases from class features, so that they map over to the undead monstrosity you end up with), and enjoy your ECL OMFG character!