So I've been playing this as my main deck and have been extremely successful with it thus far. The name is of course a reference to the fact that Space Marines allying with Tau would be quite controversial in many (if not most!) elements of the Imperium.

Warlord

1x Captain Cato Sicarius



Armies

4x Sicarius's Chosen

3x Eager Recruit.

3x Earth Caste Technician

3x Vior'la Marksman

3x Void Pirate

3x Vash'ya Trailblazer

3x Tactical Squad Cardinis

3x Honored Librarian

2x Veteran Brother Maxos

2x Daring Assault Squad

2x Land Raider

1x Ultramarines Dreadnought

Attachments

1x Tallassarian Tempest Blade

3x Ion Rifle

3x Promotion



Events

2x The Fury of Sicarius

3x Indomitable

3x Drop Pod Assault



Supports

1x Cato's Stronghold

2x Fortress-Monastery





This deck is built on the foundations outlined by my Basic Deckbuilding for the Command Struggle article. Ordinarily, Space Marines are weak in the command struggle. However, Tau allies offer many options for improving this, making them a natural choice. Earth Caste Technician, Vash'ya Trailblazer, and Void Pirate provide great capping abilities, while Vior'la Marksman replaces 10th Company Scout to provide a bit of Ranged support (especially with Ion Rifle) and a more resilient body. You then use your strong core of Space Marine units to carry the day, bolstered by all the combat tricks Marines have to offer.

You'll note that this deck doesn't run Exterminatus-- simply put, that's because it hasn't seemed particularly relevant thus far. I may test 1-2 copies of this, but thus far it's proven quite unnecessary. Similarly, No Mercy doesn't seem necessary, especially given that Sicarius often wants to be in the thick of things himself.

One problem I've found with this deck is that it's actually proven quite frustrating for some opponents. Simply put, Space Marine combat tricks are extremely strong, and I have near-maximum copies of all of them (I opted for 2x instead of 3x Veteran Brother Maxos but have 3x Eager Recruit, Indomitable, and Drop Pod Assault). Sicarius's Chosen and Indomitable are often exceptionally annoying for opponents, as they can negate very strong units for a cheap price.

I'm not entirely certain what the best way to counter this sort of deck is. It has five good AoE units to deal with Dark Eldar rush decks, two Land Raiders to counter strong card effects like Archon's Terror or Zarathur's Flamers, and the general-purpose tricks to deal with most other units someone can field. One weakness is that I don't have any Flying units and removed Godwyn-Pattern Bolter quite some time ago-- however, the current cardpool lacks solid midrange Flying units, with only the Assault Valkyrie really filling that role. I'd be interested in hearing thoughts on what the best way to counter this style of versatile SM deck is-- thus far, I haven't really run into any matchups that I would consider particularly unfavorable, though I'm sure that may change.