Josip Broz Tito
Tito returned to Moscow in August 1936, soon after the outbreak of the Spanish Civil War. At the time, the Great Purge was underway, and foreign communists like Tito and his Yugoslav compatriots were particularly vulnerable. Despite a laudatory report written by Tito about the veteran Yugoslav communist Filip Filipović, Filipović was arrested and shot by the Soviet secret police, the NKVD. However, before the Purge really began to erode the ranks of the Yugoslav communists in Moscow, Tito was sent back to Yugoslavia with a new mission, to recruit volunteers for the International Brigades being raised to fight on the Republican side in the Spanish Civil War. Travelling via Vienna, he reached the coastal port city of Split in December 1936. According to the Croatian historian Ivo Banac, the reason Tito was sent back to Yugoslavia by the Comintern was in order to purge the CPY. An initial attempt to send 500 volunteers to Spain by ship failed utterly, with nearly all the communist volunteers being arrested and imprisoned. Tito then travelled to Paris, where he arranged the travel of volunteers to France under the cover of attending the Paris Exhibition. Once in France, the volunteers simply crossed the Pyrenees to Spain. In all, he sent 1,192 men to fight in the war, but only 330 came from Yugoslavia, the rest being expatriates in France, Belgium, the U.S. and Canada. Less than half were communists, and the rest were social-democrats and anti-fascists of various hues. Of the total, 671 were killed in the fighting and another 300 were wounded. Tito himself never went to Spain, despite later claims that he had. Between May and August 1937, Tito travelled several times between Paris and Zagreb organising the movement of volunteers and creating a separate Communist Party of Croatia. The new party was inaugurated at a conference at Samobor on the outskirts of Zagreb on 1–2 August 1937. 
Josip Broz Tito
In June 1937, Gorkić was summoned to Moscow, where he was arrested, and after months of NKVD interrogation, he was shot. According to Banac, Gorkić was killed on Stalin's orders. West concludes that despite being in competition with men like Gorkić for the leadership of the CPY, it was not in Tito's character to have innocent people sent to their deaths. Tito then received a message from the Politburo of the CPY to join them in Paris. In August 1937 he became acting General Secretary of the CPY. He later explained that he survived the Purge by staying out of Spain where the NKVD was active, and also by avoiding visiting the Soviet Union as much as possible. When first appointed as general secretary, he avoided travelling to Moscow by insisting that he needed to deal with some indiscipline in the CPY in Paris. He also promoted the idea that the upper echelons of the CPY should be sharing the dangers of underground resistance within the country. He developed a new, younger leadership team that was loyal to him, including the Slovene Kardelj, the Serb, Aleksandar Ranković, and the Montenegrin, Milovan Đilas. In December 1937, Tito arranged for a demonstration to greet the French foreign minister when he visited Belgrade, expressing solidarity with the French against Nazi Germany. The protest march numbered 30,000 and turned into a protest against the neutrality policy of the Stojadinović government. It was eventually broken up by the police. In March 1938 Tito returned to Yugoslavia from Paris. Hearing a rumour that his opponents within the CPY had tipped off the police, he travelled to Belgrade rather than Zagreb and used a different passport. While in Belgrade he stayed with a young intellectual, Vladimir Dedijer, who was a friend of Đilas. Arriving in Yugoslavia a few days ahead of the "Anschluss" between Nazi Germany and Austria, he made an appeal condemning it, in which the CPY was joined by the Social Democrats and trade unions. In June, Tito wrote to the Comintern suggesting that he should visit Moscow. He waited in Paris for two months for his Soviet visa before travelling to Moscow via Copenhagen. He arrived in Moscow on 24 August.

Question: Who became Secretary-General of the CPY after the prior one was murdered?
Answer: Tito