Quick technical correction. The "deep web" is a set of library, university, hospital, and government servers, where data is stored, but isn't searchable or indexed by any public search engines. The deep web is legal, just inaccessible to the public. TSA's "no fly" list, universities' medical journals, or the US hospitals' Epic portal are on the deep web. You have to buy a subscription or work in those industries to get access.
What you're thinking of is the "dark web", where illegal products are bought and sold; Tor browsers and Onion routing are used to hide the dark web. Now, the dark web just might have fentanyl, along with much sicker things most of us can't even imagine. If you know the movie "Hostel", its Elite Hunting Club was hosted on the dark web. The "regular" counterpart to those two webs is the "clear web" or "surface web".
Source: I work with computers, I just had to say it.