Your list has been selected for inclusion in the Big List of Eldar Lists project! You will be receiving a score from me using the 5/5 system as outlined in the Big List of Eldar Lists in the stickies on this board. Following is the critique I have given:
Background: 1
For starters, I'd like to mention how much I like the idea of lists that are not take-all-comers. It actually makes me incredibly happy to see these lists because they are so suited to campaign and narrative play, and if a player is willing to accept the challenges inherent in playing it against an army it wasn't designed to face, and play it theatrically, they tend to make for awesome games. You've included just enough background to give the feel and purpose of the army, and get the point across. In a short space you've done an excellent job. I hope to see more players with these sort of lists that are geared for campaign rather than tournament play.
Composition: 1
Even though the troops section is a bit barren, this list is composed to fit the theme and tactics outlined in the background. It is literally the defensive cordon around a bunch of guns as described. It may not be made to be tournament-style, but the dual prisms and the vypers add some AT power and some ability to gun for hordes, while the guided walkers are quite nasty. The list is composed to handle alternative missions quite well. You have selected units that fill more roles than just horde killing, and while sticking to your theme, you should still have a chance against enemies that are not ideal. Furthermore, since you aim to keep shooting even after the enemy hits your line, investing in shooty vehicles to fire over combat is a good move. Because the list sticks so closely to the goals you've outlined, it scores rather nicely in this area.
Utility: 1
Almost every point spent is spent on more guns, with the goal of shooting a horde to pieces before it can reach your line. Even the scatterlaser upgrade on the vypers is purposeful, as it allows you to shoot at greater range early on and then up the fire as the enemy reach the threat zone. Embolden makes sense as these guardians will be near the table edge and you don't want them to run. I don't know if there's a single wasted point in this army.
Flexibility: .5
Within your theme you have few worries; when this list is used in its intended environment vs its intended opposition it should fare rather well, with most units capable of breaking any potential target presented by the Orks. That being said, there is basically only one way to play it, and this, coupled with unit duplication, creates two vulnerabilities that are particularly dangerous and which will have to be watched out for.
Ingenuity: .5
While you've got a sound strategy and a picture of how you think the battle will go, it would be good to add some information on how you intend to make it go that way. You need to hold off assault elements until they can be broken effectively; mention the application of the Fire Prisms as an assault barrier in this regard [same as the vypers]. You have divination; consider some of the fun you could have with that. Also consider the possibility of the different swarm styles you might face - a pure infantry horde may still give this list problems if it hits your lines [111 Orks and 3 power fists = 1000 points], so plan for it. Also, while you have a good defensive cordon, if you can afford to break your line when doing so confers an advantage, that is also something to consider. Overall you have a clear idea of what you want to do, just consider how you can guarantee that you get to do it.
Total Score: 4.0, for an excellent list
Nothing makes me happier than themed lists to be used in themed games. The idea of Eldrad holding a defensive cordon against a horde of Orks makes for an excellent image, and the idea of two armies that are aware of this theme in advance and tailored to the mission in question with knowledge of their opponent makes for a fierce match indeed. Not every army needs to be designed to face every enemy. Good job!