This is what
@Seneschal talks about:
From druchii.net:
Essentially, you take a specific set of units. Those units do not count toward your allowances for your army (as in, they aren't core, special, rare, heroes or lords for your point limits, though they count against your total points). For taking those units you get a couple free special rules. Finally, you can theoretically take as many formations as you can afford, though in this case you are limited by the special characters.
In this case, one of the special rules for these formations is that if you take them you aren't forced to take 25% of your total points in core units, as it would be the case normally.
For example, let's say you take the dwarven battlescroll. Thorgrim, Ungrim, Josef, 10 Hammerers, 10 Rangers, 10 Irondrakes and 3 x 10 Slayers are 2030 pts. If you are playing at 2500, you can still take it, as you are not forced to take any core as long as this battlescroll is in your army, and you could spend the remaining 470 pts however you wanted, as long as you meet the normal percentages, barring core.
Quote:
First, all of them allow you to avoid taking any core if you take them in your army (they are effectively the core).
Skavens:
Thanquol's uprising: Thanquol & boneripper, Verminking, clanrats, stormfiends, gutter runners and giant rats, and a unit made of warlock engineers, only one of which can be upgraded to wizard or take magic items. Units in the formation close to Thanquol are unbreakable, and the warlock unit has some advantages when casting Warp Lighting.
Clawpack of Mors: Queek, Stormvermins, 2 units of clanrats, jezzails and globadiers. Units in the formation close to Queek reroll charges, and once per battle shooting units may shoot twice.
Dwarf Throng: Thorgrim, Ungrim Incarnate of fire, Bugman, Hammerers, Rangers, Irondrakes and 3 units of slayers. Slayers reroll to hit with deathblow attacks, and can reroll 1s to wound and armor saves with core units in the whole army.
Red Host: Tehenhauin, Skink Chief, Skink Cohort, Salamanders, Chameleons, 2 Bastiladons, Riperdactyls. Hatred (Skavens), arcs on bastiladons deal 3D6 instead of 2D6, the skink and the skink cohort have +1WS
Defenders of Middenheim
Defenders of Middenheim: Valten, Battle Wizard Lord, Grand Master, Knigtly order, Halberdiers with 2 crossbowmen units as detachments, free company, swordsmen. The wizard knows a new spell, valten and units close to him are unbreakable, and as long as he is alive everyone in the empire list is stubborn.