SUMMONINGBuy Daemon packs as you would normally. Almost every Chaos player does anyway.
Buy a Mark of Chaos for the additional points cost and assign it to an appropriate stand. A Champion or other HQ unit.
Marked unit can move as it normally does and gains all the benefits of the Mark. Additionally it may summon one pack (one pack only per battle) at the end of its movement activation. It may not summon if it was engaged by an enemy in close combat. After it has summoned it may not do nothing else that turn. No Psychic attack/defence and no firing.
To summon roll a D6. On a 4+ the pack has been successfully called from the warp. Place one unit within 10cm? of the Marked unit. Roll to scatter this summoned unit and then place the rest of the pack within formation of the first, but no further than 15cm? from the summoning unit. (Ooo, it's getting messy ÂÂ

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The units summoned may be placed in close combat if within ranged and are assumed to have Advance Orders? (I would have preferred Charge but see your point of not extending the movement way beyond breaking point)
Once summoned the pack acts as a normal detachment.
I need to tidy this up and find a way for it to be clear but not as long.