A Fire Prism is an Eldar heavy anti-grav tank that looks exactly the same as a Falcon except for one minor detail: the prism cannon mounted on its top which is capable of wrecking your prized Predator tank in moments. The cannon focuses a beam of intense light within itself and then unleashes it in a blast of glory which can melt lesser tanks to pieces.
In Dawn of War the Fire Prism was hideously overpowered and beat the crap out of your squads and even other tanks mercilessly. If you had a squad of these they would fire through your forces, sending men, limbs and terrified kittens flying through the air, and DOW2 gives the Prism the ability to switch between anti-tank and anti-infantry settings just in case you really want to rub it in your opponent's face. But then, as we have established, nothing involving Eldar in DoW was particularly fair...
On the Tabletop
With T7, 12 wounds and a 3+ save paired with 16" movement a Fire Prism can move around quickly to take itself out of the enemy field of fire and take some fire it can't avoid. It comes equipped with a twin shuriken catapult and the main attraction, the Prism Cannon. No longer able to link up its shots to deal extra damage, the Prism Cannon has three firing modes:
The three firing modes all have their uses. The first is a potent anti-MEQ shot that can really thin out enemy units. But this is only to be used when no other targets are available, because the Fire Prism is a real tank hunter. The second shot is powerful against just about any type of vehicle and even Terminators in a pinch. The third shot will upon hitting a target really ruin its day: with S12 it's all but guaranteed to wound and it won't allow any saves either.
While these are formiddable stat lines they suffer from the fact that they're all Heavy. This makes the 5-point Crystal Targeting Matrix all but mandatory, but do note that it only works against the closest unit. And while with a BS 3+ this is not the biggest of issues, when it drops to 6 Wounds or less its BS starts to suffer, making the other firing modes more likely to hit. The difference between Focussed and Lance is not that big, and you're more likely to hit with at least one of the shots for Focussed. As for other upgrades, the Star Engines and Vectored Engines work well together but at 20 points require some planning, and Spirit Stones are useful but up your cost by another 10 points. The Fire Serpent can work against infantry (and is better at this with a shuriken cannon instead of twin catapults, even if it has 1 shot less), but it really shines at hunting vehicles and monsters. And while an unupgraded Fire Prism is 168 points all these upgrades will bump it up to 203, which you should keep in mind. Still, the Fire Prism is one hell of a tank hunter and outshines the more expensive Falcon. It is more expensive, less tough than and lacks the transport capability of the Wave Serpent though, and you should consider whether you want the target flexibility of the Fire Prism or the utility of the Wave Serpent to supplement your army.
The Fire Prism isn't radically improved when using Power Ratings. All the upgrades are no-brainers at this point and it can still very well hold its own. At 7 points it's two points cheaper than the Falcon, which can field weaker but more reliable anti-tank weapons and can transport squads of Aspect Warriors. It is matched in price by the Wave Serpent, which can be both a potent anti-infantry platform and a surprisingly useful anti-tank vehicle with twin Bright Lances. While the Fire Prism is strictly better at that job, the Wave serpent's toughness and transport capabilities are more than able to make this a viable alternative.