Pure Heroin is fairly potent on its own, it's 2-4 times more potent than Morphine. The only problem is the manner Heroin is usually gotten, with drug dealers. Heroin from drug dealers, you can never really tell what kind of batch you're getting. If you're hoping to CTB and it gets laced with Fentanyl, then you're in luck because Fentanyl is even stronger than Heroin (it's at minimum 50 times more potent than Morphine). But at the same time, that batch would also be laced with a bunch of other crap not useful for aiding in CTBing.
Also almost all dark net markets have banned both the selling of Fentanyl as well as lacing Fentanyl in other products, so I wouldn't bet on hoping to get Fentanyl laced in other products. I've heard China White Heroin can still laced with Fentanyl, but unless if you're willing to go through the trouble of sending a small sample off to Energy Control International to test for Fentanyl I wouldn't bother. Also I only one of 1 dark net market that's apparently still allowing 1 individual to sell pure Fentanyl on there, but whether it's legit or not I don't know.