This is a great conversion "trick" that I've seen used a couple of times on the internet. The first time I saw it at was as the Shadow Hunter Cruiser displayed at Dropship 40K. They have, by far, done the best job as well. I decided that I simply must build one, so I did, and made stats for it as well. Here's a step by step walktrough on how to build your own.

The main things you'll need for this conversion are the main body of a 40K Eldar Support Weapons Platform (the part code is 010407701), and an Eldar cruiser of your choice, both the Shadow and the Eclipse work equally well. If you're a perfectionist like me, you might want some green stuff as well.

First off, you can start by trimming the grav jets off the Weapons Platform bit. I used a jewlers saw, but if you've got a Dremmel tool you can do a rough cut with some metal shears and clean the edges up after. From the top it isn't as obvious as to where you want to cut, but on the underside you'll see that there are some thicker ridges that you'll probably want to work around.

Next, get rid the three little bumps on the back of the platform, they'll impede the assembly later. File (or Dremmel, you lucky bum) down the cut edges so that they're relatively smooth.

There's an obvious hole in the middle of this bit where the gunner's pivot step is supposed to go. To cover/decorate this, the folks at Dropship 40K cut the top off the Shadow Weaver (part code 010407706) that comes with the platform. I lament to waste good bits in this way (I have a 40K Eldar playing friend who wanted to buy it), so I merely used a small plastic dome I had in my bits box.

The basic assembly should be pretty obvious: put the weapons support platform were the cruiser's prow bit would usuallt go (I used green stuff to attatch them, since the bottom of the support platform isn't quite flat). As for the cruiser's wings, I decided not to attatch them in the usual place, but in a groove along the side. This makes the wings splayed out, and makes the ship look bigger. Again, green stuff was used due to the unlevel surfaces.

There's another option to think about now. Our Dropship 40K friends put a pair of Nightshade Class Destroyer wings on the sides of the main body. This helps distract from the place where you cut the jets off, and its a neat converstion you can use on any Eldar captial ship. I didn't bother with this because I don't have the cash to buy or order a blister of escorts.

The Dropship Shadow Hunter has what looks like a big pulsar lance on the front. I imagine they chopped up another Eldar escort to make this structure. I wanted to keep with Dropship's original idea, which is why I added a big prow pulsar lance in the rules I made. I had an extra 40K Scatter Laser (part code 010407903) which I decided would look good. I cut off the barrel of the weapon, as well as a small oval (see picture). I then filed both ends of the barrel, and affixed the oval to the level end. The angled end I later attatched to the angled surface on the front of the weapons platform bit, between the two forward blades (again, green stuff was used).

After all the glue and putty was given some good, long drying time, I started on the painting. I decided that an expensive piece of hardware like this ship should belong to a craftworld fleet. I chose Ulthwé's color scheme: Black undercoat, off-white highlights, red gems. I believe it turned out quite nicely. Good luck with yours!