".... international law supported and colluded in many of the worst human rights atrocities, including the Atlantic Slave Trade and colonialism ..."
Frans Viljoen, 'International Human Rights Law: A Short History' UN Chronicle (2009) 46 [http://unchronicle.un.org/article/international-human-rights-law-short-history/
The Routledge History of Human Rights by 
The History of Human Rights : from ancient times to the globalization era by 
Rights and Civilizations : a history and philosophy of international law by ".... It was only in the nineteenth century that the international community adopted a treaty abolishing slavery.'
Frans Viljoen, 'International Human Rights Law: A Short History' UN Chronicle (2009) 46 [http://unchronicle.un.org/article/international-human-rights-law-short-history/
Declaration of the Eight Courts Relative to the Universal Abolition of the Slave Trade of 8 February 1815 (63 CTS 473)
(The eight were Austria, France, Great Britain, Portugal, Prussia, Russia, Spain, and Sweden)
Redman, Renee C ‘The League of Nations and the Right to Be Free From Enslavement: The First Human Right to Be Recognized as Customary International Law’ (1994) 70 Chicago-Kent Law Review 759–802 (Holders of an Oxford SSO can read this article online via HeinOnline, Westlaw, or LexisLibrary.)
J Allain ‘Slavery and the League of Nations: Ethiopia as a Civilised Nation’ (2006) 8 Journal of the History of International Law 213–44. Holders of an Oxford SSO can read this article online via HeinOnline or Brill.)
Subject searches to use in SOLO
Slavery -- Law and legislation -- History
Antislavery movements -- History
or if you have a geographic interest you can adapt eg
Antislavery movements -- Great Britain -- History
Human rights -- History
Slave Trade and the Origins of International Human Rights Law by 
Suppression of the Atlantic Slave Trade: British policies, practices and representations of naval coercion by After the First World War, tentative attempts were made to establish a human rights system under the League of Nations. For example, a Minority Committee was established to hear complaints from minorities, and a Mandates Commission was put in place to deal with individual petitions of persons living in mandate territories. However, these attempts had not been very successful and came to an abrupt end when the Second World War erupted.'
Frans Viljoen, 'International Human Rights Law: A Short History' UN Chronicle (2009) 46 [http://unchronicle.un.org/article/international-human-rights-law-short-history/
'The first international legal standards were adopted under the auspices of the International Labour Organization (ILO), which was founded in 1919 as part of the Peace Treaty of Versailles. ILO is meant to protect the rights of workers in an ever-industrializing world.'
Frans Viljoen, 'International Human Rights Law: A Short History' UN Chronicle (2009) 46 [http://unchronicle.un.org/article/international-human-rights-law-short-history/
Human Rights, Development and Decolonization : the International Labour Organization, 1940-70 by 
International Labour Law by