It was linked to a growing number of deaths in the USA, Canada and Europe before a similar pattern began to emerge in the UK.
There have been several such deaths in Yorkshire and the Humber, prompting a series of public warnings about heroin laced with the drug.
Fentanyl is a synthetic opioid, usually presented in a gel or tablet form.
It has legitimate medical use as a painkiller and is prescribed in a number of formats, including patches and nasal sprays.
However, the drug is highly toxic and as little as 0.002g of fentanyl (within a typical 0.1g heroin deal) is potentially fatal.
Fentanyl is around 50 times more potent than heroin and 100 times more potent than morphine.
There are a number of 'analogues' of fentanyl - compounds with a similar structure but varying potency.
They include carfentanyl, which is not intended for human consumption and is generally used to anaesthetise large animals such as elephants.
Carfentanyl is as much as 10,000 times stronger than street heroin, meaning the equivalent of a few grains within a typical heroin deal constitutes as lethal dose.
In a briefing, the NCA said fentanyl and its analogues may be mixed with heroin in street deals - but the drugs were also being bought on the dark web by people who wished to experience their effect as drugs in their own right.
Illicit supply from Chinese manufacturers and distributors is a prime source for both synthetic opioids and the chemicals used to manufacture them.
To date, 74 drug related deaths in the UK since December 2016 are known to be linked to fentanyl or one of its analogues.
This has increased from the figure of 60 published in July after cases were revisited by the coroner or secondary toxicology tests were carried out.
The NCA is working with other organisations nationally and internationally to identify key suppliers operating via the dark web.
Police operations resulting from this work include a raid on an illegal drugs lab in the Morley area of Leeds.
Three people were charged with conspiracy to supply and export class A drugs.