Nikolai Ivanovich Yezhov, IPA: [njIka'laj jI'zof]; May 1, 1895 - February 4, 1940) was a Soviet secret police official under Joseph Stalin who was head of the NKVD from 1936 to 1938, during the most active period of the Great Purge. Having presided over mass arrests and executions during the Great Purge, Yezhov fell from Stalin's favour and power. He was arrested, confessed to a range of anti-Soviet activity, later claimed he was tortured into making the confessions, and was executed in 1940.

Yezhov was appointed People's Commissar for Water Transport on April 6, 1938. Though he retained his other posts, his role as grand inquisitor and extractor of confessions gradually diminished as Stalin retreated from the worst excesses of the Great Purge.  By saddling him with the extra job, Stalin killed two birds with one stone: Yezhov could correct the water transportation situation with tough Chekist methods, and his transfer to the terra incognita of economic tasks would leave him less time for the NKVD and weaken his position there, thus creating the possibility that in due course he could be removed from the leadership of the punitive apparatus and replaced by fresh people.  Contrary to Stalin's expectations, the vast number of party officials and military officers lost during Yezhov's purges had been only partially made good by replacement with trusted Stalinist functionaries, and he eventually recognized that the disruption was severely affecting the country's ability to coordinate industrial production and defend its borders from the growing threat of Nazi Germany. Yezhov had accomplished Stalin's intended task for the Great Purge: the public liquidation of the last of his Old Bolshevik political rivals and the elimination of any possibility of "disloyal elements" or "fifth columnists" within the Soviet military and government prior to the onset of war with Germany. From Stalin's perspective, Yezhov (like Yagoda) had served his purpose but had seen too much and wielded too much power to be allowed to live. The defection to Japan of the Far Eastern NKVD chief, Genrikh Lyushkov on June 13, 1938, rightly worried Yezhov, who had protected Lyushkov from the purges and feared he would be blamed.

how long was he in power before the fall from power?
the possibility that in due course he could be removed from the leadership of the punitive apparatus and replaced by fresh people.