Stormvermin are what skavenkind produces when it feeds some of its children properly: black-furred brutes a head taller than any clanrat, drilled to fight in ordered blocks and equipped with real armor and cruelly hooked halberds. They serve as warlords' bodyguards, enforcers of what passes for order in the burrows, and the unbending core of armies otherwise built from panic.
Privilege breeds arrogance, and arrogance breeds something almost like courage — a stormvermin will hold ground long after the clanrats have fled, if only because failure would mean rejoining the ranks of the hungry. Almost, however, remains the operative word: they are still skaven, and no oath sworn in the under-empire has ever survived contact with a losing battle.