The REAL Fire Prisms review.
The Review -
Like all other 'heavy' Eldar tanks, Fire Prisms are a fast tank with skimmer status with AV12/12/10 - i.e. pretty durable, mobile and capable of shooting whilst on the move. They come with a very big gun which has two firing modes - high strength/low ap small blasts or middling strength/middling AP large blast which works best when there are two Fire Prisms around as their shots get even better and come with twin-linked.
This provides the Eldar army with something they don't really have - good blasts which force opponents to spread out or get smacked pretty harshly. This is obviously the most effective when combined with tank shocks, recently disembarked units or units recently engaged in combat. With the ability to effectively fire the linked blast from two different points on the tabletop, denying cover saves to such units can also be easier than with just a single tank.
However, like most big gun tanks, Fire Prisms suffer from one gun syndrome and the tank can be effectively useless after it loses its main weapon (its not, but it is a downside). Fire Prisms do have a pretty significant advantage here though - unlike many single gun tanks they are a fast skimmer which means their options of ramming, blocking and tank shocking are much bigger and therefore has greater tactical advantage.
Most of the upgrades aren't needed for the Fire Prism. Spirit Stones are generally an okay investment and keep the Prism moving and the Shuriken Cannon gives the Prism some extra options when the main gun is taken off. That's about it. People love to run Holo-fields on these guys but its not worth it and makes a slightly expensive unit insanely expensive. Yes, it makes the unit more durable but you're more likely to lose your gun - the whole point of the tank. Over two of these you could nearly buy another tank if you drop the Holo-field/Spirit Stone combination.
Potential Uses -
Fire Prisms work best in pairs as they are able to bring more utility to the table with the combined shot - a S6 AP3 large blast or S10 AP1 small blast with twin-linking is pretty nice though quite expensive at 230 points and one weapon destroyed result nullifies this. That being said, they are very useful at providing extra anti-infantry firepower to blast whatever you need apart and can do so with one super blast or two smaller ones. Running a third one adds that extra bit of redundancy but you'll often find better uses for that extra Heavy SUpport slot. You can also run one but you lose some utility without the combined shot.
Beyond this, Fire Prisms should be taking shots at clumped infantry - however that may come apart. Use the speed and range of the Prisms combined with the linking ability to get the best possible angles whilst avoiding reprisal - outside of Vypers, Prisms will be the weakest Eldar tanks out there. Remember as well, they are not effective tank hunts. S10 AP1 looks nice, particularly with twin-linked but that's 230 points - it is certainly a use early game when there's no infantry to fire upon but once there are better infantry targets, that's the Prism's main goal.
Potential Changes -
One gun tanks need a change overall from an edition change and if this happens to make them more viable (i.e. weapon destroyed is random, larger weapons require more 'weapon destroyeds' until they are actually removed, etc.) well the Fire Prisms is sitting at a pretty solid place. Regardless, a slight points reduction wouldn't be amiss and well really, that's about it that's needed assuming such a fix is made.
If not, some sort of extra defensive measure on their main gun is important to keep them useful in light of improving choices around them in the Heavy Support slot. Whether such changes are similar to what is seen above but simply for the Fire Prism or not... well any other ideas here? Spirit Stones could evolve to being able to ignore one weapon destroyed effect and then be destroyed themselves (i.e. burnable armor).
Making them more useful as a single tank would also be nice - i.e. up the strength/AP of the secondary blast just a little bit so you don't really NEED that 2nd tank to make them really useful.
The Fire Prism is a pretty decent tank currently though it suffers from one gun syndrome. It works better in pairs as the utility is increased in what its main weapon can do and also force opponents to be wary of potentially low AP small and large blasts. Using them to drop a reliable or multiple low AP blasts on opponents from a variety of angles can really help the Eldar anti-infantry potential out. Any changes, whether through main rules or Eldar specific, which make the one gun syndrome less of a liability and making the Prism more independent (not needing two to be really effective) is going to make them a better choice overall.
In the end, keep them cheap without unnecessary upgrades and use them to force your opponent to spread out and provide some low AP firepower.