Greetings fellow Shas'O,
There is one phase we all have to sit down quietly and wait till the opponent finishes off our troops with some wicked wizardry, or makes their already powerful units almost invincible and we can not do anything about it. So while of course, we could bring some wicked wizardry of our own in a form of the allies, but wouldn't it be better to piss off our psyker-dependent opponents if we just completely broke down their whole strategy by turning their powes off and then finishing them off with our superior technology? Of course it would.
So as far as I know, there are three options for us to pick.
The most obvious pick out of them all would be the Culexus Assassin - he's quite costly, but he has great statline, is very independent which is great concerning the Imperium is not our ally, and is very hard to hit with his BS1/WS1 secial rule. And of course the psyk-out grenades are always useful.
The other pick is Ordo Hereticus Inquisitor - he has a Null Rod, which makes his unit invincible to Psychic Powers. He's almost three times cheaper than Culexus Assassin, but he has a statline only slightly better than that of a Fire Warrior, so we'd have to bring a whole squad of Inquisitorial Henchman for him to be of any use. And it still won't stop the psykers from attacking our other units.
And then comes the Burning of Prospero with the Sisters of Silence, they have a better basic statline than the Inqusitor, believe it or not, and all of them have a 12" null-bubble we all love Culexus for. The only malus of them is that their statline is still not that impressive, the same as of the Sisters of Battle, so they're pretty squishy.
And the last item that would help us is obviously the FSE Talisan of Arthas-Moloch, but it only helps us, though greatly, to ward off spells that are directed towards our commander and his unit. That doesn't stop the enemy from summonning hordes of daemons or makes them suddenly visible for us again (markerlights are great, sure, but sometimes this is just not enough).
The most obvious downsides of both the Talisman and the Inquisitor is that they're both affecting only one unit, and while it's something, especially if you go for a blob, it's not going to help that much against enemy's blessings nor summonnings.
While Culexus is great, as he's completely independent with all of his special rules and much harder to kill, he's still prone to getting instakilled with a lucky melta shot or he's going to get chopped to death with his 4++ being his only save. His Animus Speculum might be useful with AP1, but it's still not something great. And this all comes at a heavy price - 140 pts and a detachment slot, unless you bring all the Assassins on the table, which might not be always the birightest idea.
And then I found the new Sisters of Silence rule, which sounds like the best alternative to Culexus. They are actually better than him, as every sister in the squad creates a null-zone of her own, effectively improving its range at least twice. They have a better basic save than Culexus (which makes them more prone to grav-guns, sure, but it's still a better save). No inv though so they have to watch out for the low-AP weapons. These are however 5 models, not just one, they come at half the price of a Culexus and their formation's special rule makes life for the psykers even nastier. They still do take one detachment from your army, but it's, once again, not just a one model and they all come armed with a boltgun and psyk-out grenades. The worst part is that they can't take a dedicated transport, so unless you're picking another allied detachment, these squishy troops have to go 6" ahead, next to or behind your army.
So Shas'O, how do these options look for you? What would be the best pick for a Tau Commander? Have I missed anything?
Keep a glorious fight against these supersticious witches and bring them all the light of the Greater Good!
Prosper, as Tau shall.