INS Chakra was formally the Nerpa, a Russian Akula II class submarine that was supposed to be turned over to India (which is leasing it) three years ago. The main reason for the delay was a safety issue. Four years ago, during sea trials, there was an equipment failure that killed 20 sailors and shipyard workers aboard the Nerpa. This delayed sea trials for many months and the Russians found more items that needed attention. These additional inspections and repairs continued until quite recently. India is paying $90 million a year for ten years to lease the Nerpa, an 8,100 ton Russian sub that was then renamed INS Chakra (the same name used by the Charlie class Russian sub India leased from 1988-91) by the Indians.
There have been many reasons for getting this sub from Russia. Back in 2010 the Indian crew, after more than a year of training, found that they were not fully prepared to take over the sub. The crew required another six months of training. The Russians were being blamed, partly because they were in charge of the training and partly because they recently made a lot of internal changes to the Nerpa. But Indians also admit that all their veteran nuclear submarine sailors (who manned a leased Russian nuclear sub from 1988-91) were retired and the difficulties of learning how to run a nuclear boat were underestimated.
The Nerpa was built for this Indian deal and finally completed its sea trials and was accepted into Russian service in late 2009. India was supposed to take possession in May 2010, but there were more delays, mainly because of the accidental activation of the fire extinguisher system and death of twenty on board. There were 208 people aboard the sub at the time, most of them navy and shipyard personnel there to closely monitor all aspects of the sub as it made its first dives and other maneuvers. The source of the fatal accident was poor design and construction of the safety systems. This accident led to sailors and shipyard technicians being fearful of going to sea on the boat. So the sea trials were delayed, even after repairs were made.
The post-accident modifications on the Nerpa cost $65 million. The lease arrangement has India paying $178,000 a day, for ten years, for use of the sub. The 7,000 ton Akula II requires a crew of 73 highly trained sailors. Over a hundred Indian sailors have undergone training to run the boat.
It was Indian money that enabled Russia to complete construction on at least two Akulas. These boats were less than half finished at the end of the Cold War. This was another aftereffect of the collapse of the Soviet Union. Several major shipbuilding projects were basically put on hold (which still cost a lot of money), in the hopes that something would turn up. In this case, it was Indians with lots of cash.
Traditionally, when a new ship loses a lot of people during sea trials it is regarded as "cursed" and unlucky. Sailors can be superstitious, especially when there are dead bodies involved. It's not known if India will have any problems with this.
India has designed and built its own nuclear sub but the first one is basically a development craft, and mass production of Indian designed nuclear subs is still 5-10 years away. The unlucky Russian sub will enable India to train more nuclear sub sailors in the meantime.