6th Edition 40K Unholy Alliances

There are bitter enemies in the grim Darkness of the 40K universe, and some are so diametrically opposed that they would sooner kill you than look at you.

There are those commanders willing to trade CERTAIN obliteration at the cost of POSSIBLE obliteration.  Faced with an enemy so immense that they themselves, nor those they might ally with, can possibly survive against, they will create unholy alliances.  The outcome can only be betrayal in the end.  Nonetheless, the slim chance of at least some of their forces surviving is enough for both forces to dance with the enemy.  Better than no chance at all.

I am going to go over four of the most Unholy alliances.  These are not commonly seen but hold devastating possibilities.

Blood Angels with Necrons: 
Blood Angel forces are strong on the Assault but lack long range firepower if they are to maintain their potency in that arena.  Two things make an assault army particularly deadly.  The first is surprise.  If an assault army can safely navigate to its target via terrain or other means, it can be like a tsunami when it arrives.  No amount of Over watch will save you if you haven't whittled the forces of the Blood Angels.  Enter the Solar Pulse.  Coupled with the speed of the Blood Angels attack, and feel no pain, this is a powerful combination.  Spared the usual attrition they would face against anti-MEQ weaponry because of the saves they get from Stealth and Shrouding rules and Feel No Pain, the army can gather its strength and punish the enemy.  The Storm Lord, while expensive, ensures two rounds of this, but just a Solar Pulse is still quite adequate.  The ideal part of this equation is that the Storm Lord really need not be anywhere near the other troops to be effective and thus will not likely trigger the need for the "One Eye Open" rule.  The Necrons can also provide anti-aircraft a little cheaper than the Blood Angels can if you want to handle that duty with Flyers.

Chaos Daemons with Orcs: 
Daemons really want to soften their landing zones.  This is a problem because if they put shooters down, their range is poor and they are squishy.  Flamers are really the one units they can count on but they have to make RISKY deep strikes in order to be at maximum effectiveness.  The combination of Daemons and Orcs gives the crazed Warp beings what they really don't have:  a fusillade of shooting with excellent range.  When in cover, it has enough survivability, plus they somewhat obviate your need for an Aegis Defense line or other anti-aircraft answer.  The Daemons work far forward of their own board edge as a rule, so the presence of the Orcs won't be likely to activate the One Eye Open Rule.  A unit of lootas can do serious damage to soften the landing zones of the important Daemons to come.  Gretchen protect the War boss and can be used as extra wounds, and speed bumps The entire investment here is about 350 points or so and the Lootas can easily replace any shooting unit you would otherwise have used except the Flamers of course.

Sisters of Battle with Tau:  
The Sisters of Battle really only have one weakness:  range.  It is a  pronounced weakness and they take terrible losses in the commission of their holy duties.  Range comes from just two units:  Retributors and Exorcists.  Exorcists are unreliable, though potentially very good at what they do.  The Retributors have just a 36 inch range with no real mobility to it.  Enter the Tau.  The Tau feature both the Command and Crisis units, both of which can carry a good amount of STR 7 long range shooting, and the Broadsides are known the multi-verse over for their obvious firepower contributions.  A command unit of Crisis suits with Missiles can go a very long way towards solving the First Blood and Rhino busting woes of Battle Sisters during the first and second round.  That allows the massive firepower of the Sisters of battle at short range to matter more.  Sisters often sacrifice units to get their enemies into good firing position and those losses are somewhat avoidable if their range were better.  Things like Manticores that terrorize the Sisters of Battle would be quickly dispatched.  Target Locks allow even one unit of Broadsides to be a considerable addition to a Sisters of Battle army and quite reliable at their jobs.  Killing broadsides is nowhere near as easy as killing the tanks either.  With Shield Drones they are very hard to disable in enough time to matter when a wall of Sisters are blocking the way and pressing forward.

Dark Eldar with Sisters of Battle: 
St. Celestine is an asset to any army and she floats happily alone anyways.  her troops are better in general than the alternatives in the Dark Eldar Codex if you have assault covered elsewhere.  So it might seem odd to do, but having a weapon like the Living Saint and some of her troops along is far from your worst alternative.  Her 2+ save can come back again and again to protect her charges if she is not willing to leave them behind, making them rock hard to get off of objectives.   Limited Acts of Faith will have almost no consequence in a sisters of Battle allied contingent and the Seraphim as Jump troops over the Dark Eldar alternative is very far from the worst you can do!  All of this adds up to a versatile addition.  The One Eye Open rule is more likely to come up with this type of Ally and that will have to be a risk you're willing to take but Dark Eldar gain a lot by adding Sisters of Battle to their long list of enslaved servants.  To have broken the will of the Living Saint would be proof to all that your Archon is ready to take on Asdrubiel Vect himself...