500 point campaign army-any ideas?

Oh man. When you say, 'a psychic power', do you mean ANY Eldar psychic power?
If you only mean the 'minor' ones, then veil of tears is great* now and decent at all levels.
*possibly game-breaking.
Other good choices might be Conceal or possibly Embolden (so any Farseer who ends up attached to the DA can reroll his psychic tests).

If you do mean any psychic power, Doom. It works amazingly with your squad because your Exarch setup doesn't involve your Exarch shooting at all, so you don't lose anything by using it. As a result, you gain hugely in wounds caused and your squad might actually do okay in assault.
Fortune or Mind War are other good choices, of course; I'd lean towards Veil of Tears over Fortune.

But yeah. No matter what you get, it's great. I'd go Doom if Farseer powers are allowed, or maybe Mind War. If they aren't allowed, Veil of Tears will be amazing all the way. If you're only allowed Warlock powers, Conceal may be too situational. Embolden as mentioned above, or Destructor. Destructor would be better in this squad than many others because you've got that cc build, and so presumably will be close enough to squads to flame them.

Lord Rahl said:

I would say using Guide would be the best as it will make for the lack of firepower on the Exarch, also when used in conjunction with Bladestorm, the unit will be crack shots and hit everytime. More shots hit = more armour saves for the opponant to fail.

Click to expand...

Doom's better, though. Guide takes a 2/3 to hit and makes it an 8/9. Your deadliness has gone up by 1/3.
Against T3 enemies, Doom is exactly as good as Guide; against T4 or higher, Doom is moderately and then considerably better (deadliness increases by a half vs T4, by two-thirds vs T5, and by around 80% vs T6 or T7) . Furthermore, this is all assuming you've only got one squad. Once you have 2+ squads of Avengers, Doom blows Guide out of the water. Basically, don't make your strong point better, make your weak point better. And of course all this is in the effort to just pile up the saves on whatever you're shooting.

The logic behind taking veil of tears is this: once you have enough Dire Avengers in serpents, you can sometimes or even often obliterate every unit in an 18" radius from where you drop them off (supposing you choose the dropoff point intelligently enough). If you do this normally, you're at risk of your avengers suffering from AP4 templates or firepower at much longer range. If you do this and one squad has Veil of Tears, you can start to pull tricks with positioning. Situational, possibly amazing, possibly useless.

Bottom line: I love Stephen's choice of Doom, it's solid everywhere.