Mega Battle (Escalation / Stronghold Assault / Apoc)...

Excuse the cheesy/bad photoshop job here; this was the start of our mega battle. Dark Angels and IG versus Blood Angels, Grey Knights and the unexpected Eldar. Additionally Orks were appearing all over the battle field from various secret (random) deployments to harass both sides.

 This was the DA & IG deployment side.

 Here some Eldar, Blood Angels, Grey Knight Terminators and a Baneblade wait for instruction.

 Here you can see the mysterious Eldar set up a kill zone to eliminate a sudden Ork threat - however in that moment the Red Thirst came over the Sanguinary Guard and they rushed into close combat before the Eldar could fire a shot.

Here are some great narratives by the other players:

Rumblings in the warp disrupted the astropathic transmission sent by Dante during the climax of their previous battle. Reports of Dark Angels treachery never reached the Ordo Hereticus.

When the Dark Angels' deception was revealed to the Grey Knights, they joined forces with the Blood Angels, and attempted to forcibly sanction their old allies. Meanwhile, Eldar forces spilled from the webway and offered to join the fight, their seers whispering warnings of a hidden menace. It is unclear what threat the Eldar psykers divined, but as the earth quaked beneath the feet of titans and the tracks of massive battle tanks, something stirred. Orks burst from the ground and began attacking anything in sight. The infestation inadvertently helped the Dark Angels and their Imperial Guard allies halt the enemy advance, and escape the planet.

Intercepted vox transmissions used veiled language and obscure codes, but Grey Knight librarians and inquisitorial code breakers were able to decipher repeated references to "the fallen."
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And another:

Incense wafted across the command chamber and through the banks of servitors. The vapor splashed into the Lord Generals tattooed face, a smirk on his lips. Hooded officers stood to his left and watched the pic caster with delight as an Imperial Baneblade detonated into the horizon. The Lord General turned to them.

"Where were they when the Orks nealry razed our world?"

"They couldnt be bothered to help such a small system but look at us now." The officers shrunk back as his voice began to rise. 

" The Imperium and these Xeno swines are being crushed by our might!" He slammed his fist on the arm of his control throne.

An officer with a grizzled beard knelt forward and saluted.

" Lord Azrael will be pleased we have repaid our debt to his glorious chapter." 

"Indeed." The Lord General lit a fragrant Tanyee cigarette and inhaled deeply. 

"Land the rest of our troops 3 clicks north of the refinery, we'll advance our infantry through the dense forests there, It'll be just like home in that mess."

The officers nodded and scrambled for their cogitor consoles. 

"This fight's just warming up" he muttered as he took another long draw off his cigarette.

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In the end commander Dante and his allies were unable to seize any of the objectives; and the treacherous Dark Angels and their conscripted IG were able to withdraw from the planet unscathed. None know if the Eldar will continue to fight alongside Dante and the Grey Knights but it's clear that their arrival portends greater threats on the horizon...