Within the interal Clan-hierarchy, there is always a bigger Rat
Thrall Clans are those Skaven Clans which have lost or given up their independence to a much stronger Clan. Whilst all Warlord clans are eager to secure treaties and pacts with more powerful clans, there are those who cannot purchase such alliances. Some Warlord clans who willingly throw in their lot with one of the four Great Clans, trading total obedience for power otherwise unobtainable. These clans, known as Thrall Clans, are in effect extensions of the Great Clans themselves.[1a]
Whether the Great Clans see these Skaven as actual (if temporary) allies or as unwitting pawns likely depends on the size and strength of the Thrall clan in question at any particular time. Unsurprisingly, many of these Thrall clans share the same ideology and goals of their masters and their armies incorporate a disproportionate number of their patron's weaponry, warriors and war-beasts. With such favours do the Thrall clans defeat their rivals and secure their own powerbase. Many tend to dwell in lairs and strongholds far from the eyes of their masters and hence they have a greater rein to pursue their own, nefarious agendas. However, it is a foolish Thrall clan that forgets its bonds of fealty altogether.[1a]