The Nightshade Interceptor is an Eldar fighter aircraft and the vehicle of choice for the Crimson Hunter Aspect Warriors. This of course means that it is an exclusive aircraft for a certain Aspect. Why in Asuryan's name someone thought it was a good idea to make the Nightshade exclusive to a single division of Eldar rather than, you know, share it with everyone else to make it a full fledged fighter is currently a mystery.
The Nightshade represents the pinnacle of Eldar aeronautic ability. They look similar in appearances to a Hemlock Wraithfighter. It is a high-speed craft designed to destroy any enemy fighter, even the advanced models of the Dark Eldar, meaning that it can function as both dedicated anti-vehicle and anti-aircraft support. And seeing as how it can keep up with Dark Eldar craft, it is quite impressive on its own. The Nightshade is most typically armed with two Bright Lances and a Pulse Laser which can be swapped for Starcannons.
Crunchwise, Crimson Hunters are like their Exarch, except even less accurate, since it doesn't re-roll 1s to hit, and is only marginally cheaper. Generally, skip - take an Exarch or don't take a Crimson Hunter at all (although a Hemlock is still better). However, if you take the Exarch, it would be more cost-effective, as for 15 more points, it gains re-rolls of 1s to hit or better. Has twin Bright Lances and a Pulse Laser, and can swap the bright lances for starcannons.
It could also re-rolls failed wounds against targets with fly, but at only BS4+ re-rolling 1s against enemies with Hard to Hit, and with low shot volume for dealing with jetbikes and jump infantry, broadly worse than just taking a Jinx Hemlock for solving the same problems. Oddly enough our dedicated fighter aircraft is worse at shooting down planes than the earth-bound Dark Reapers. As an anti-monster/vehicle or anti-MEQ/TEQ unit, however, Crimson Hunter Exarchs are not to be underestimated. The default underslung Pulse Laser is a significant threat to any multiwound models not packing storm shields and the semi-flexible loadout options give them a little more customizable utility when compared to Hemlocks or standard Hunters.