This article analyses the (post)colonial politics of UK bordering through the lens of monstrosity. Historicising contemporary bordering within colonial-era monsterisations of racialised people and their mobility, we identify four mechanisms through which migrants are constructed and policed as monsters today: animalisation, zombification, criminalisation, and barbarisation. We then examine how the state embodies monstrosity itself through border policies of deterrence and creating ‘hostile environments’. In addition to the instrumentalisation of horror, this entails extending the border’s reach domestically throughout everyday life and internationally through deportation and externalisation measures. We argue these developments embody a new form of state power, depicted as a headless, tentacled Leviathan. Doing so provides insights into monstrosity as a form of liberal statecraft, the local/global diffusion of bordering, the transnationalisation of sovereign power, and the racialisation of citizenship. It also raises important questions about the construction of border violence as a necessary and legitimate monstrosity in (post)colonial liberal societies, the everyday complicity of citizens, and the limits of efforts to humanise monsterised migrants or reform monstrous state institutions. Revealing how within liberal regimes of citizenship and humanitarianism values ‘there be monsters’, we argue, opens space for thinking about abolitionist alternatives in international politics.