Skaven never go into battle without a squirming horde of giant rats surging about their legs, swarming over enemy defenses, devouring and befouling enemy grain supplies. The packmaster herds the swarm, cares for the beasts, breeds the strongest with the strongest to create more ferocious rats, performing all of these tasks under the cruel supervision of the tribe’s verminlord, a monstrous skaven so terrifying that it rivals the power of the grey seers.
Most of the vermin that accompany a skaven tribe are giant rats. The packmaster’s bizarre husbandry practices, often combined with wicked magic, produce other, more dangerous forms of the giant rat. These ferocious rodents are used not only as troops and guards, but the larger may serve as mounts for packmasters and other exceptional skaven. To determine the stats of these rats, simply take the stas of another monster and change the name and appearance. A few examples could include blink rats, ghoul rats, horse-sized war-rats, and packs of howling worg rats.
Armor Class: 6 
Hit Dice: 2+1
Special: Crack the whip
Packmasters herd the skaven horde of rats and rat-like monsters, often while mounted on a worg rat. When a packmaster cracks its whip, vermin within 60 feet know to attack with greater ferocity, and so gain a +1 bonus to attack rolls for the next 1d4 rounds. A packmaster may not crack its whip and attack in the same round.
Armor Class: 5 
Hit Dice: 4+1
Attacks: Claws or weapon (1d6+1)
Special: +2 bonus on attack rolls, immune to fear
Rat ogres are giant, mutated skaven that combine minimal intelligence with great strength and bloodlust. They are fearless, and they attack with berserk fury with either weapons or their deadly claws.
Armor Class: 3 
Hit Dice: 6+1
Attacks: Weapon (1d6+1)
Special: Disease (75%), spells
Horrific examples of pure pestilence given form, Verminlords live only to despoil and corrupt. Each clan has a verminlord, chosen from among the ranks of the packmasters by the tribe’s grey seer, who then subjects the chosen one to horrible rituals. In the unlikely event the packmaster survives, it emerges transformed as a verminlord, a creature with great skill in combat as well as magical powers. All vermin within 60 feet of a verminlord burn with rabid hatred, causing +1 damage on attacks. The verminlord’s own attacks are very likely to leave any who survive infected with some ghastly disease. It casts spells as a 3rd-level Cleric and a 3rd-level Magic-User, but its spells may never heal or cure.