It is common practice for Clan Moulder to sell both beast packs and Packmaster handlers to the highest bidder. For this reason, Packmasters are themselves cagey and fierce warriors or, if they are not, they quickly end up as another meal for their merciless packs.
Packmasters are specially-trained Skaven who are experts at goading their charges - ferocious, half-mad creatures who can turn and attack with no warning.