Big enemy in another plane, but which one?

Not a lot of information to go off of in terms of specifics in your campaign, but an often forgot truth is that you can make up whatever you want!

Close your eyes - picture the baddie - where are they? what sort of environment are they in? Pick a plane that mimics what you envision and that's what it is! Why is it hard to travel there? It doesn't have to be written down in a splat book necessarily, it can also be something you just make up!

Going with your black diamond prompt...

Atop a plain pedestal in an otherwise empty room sits a large, opaque black diamond - completely devoid of color. As one approaches closer, within the diamond can faintly be seen hint of swirling movement in the void black diamond - almost like water chaotically swirling beneath it's surface. When touched, the diamond will transport the PC to the Plane of Smoke.

Once a stronger and more clear path between the Plane of Smoke and the Material Plane existed, but, recently a powerful planar merchant known as the Master of Ciders has cut the plane off from easy travel and only a finite number of portals currently exist, such as the black diamond, that were built specifically as links to the two planes. The black diamond is an incredibly obscure example, if not unknown entirely, but some commonly referenced links in legend include:

 - The Bytopian gate said to have been constructed by Garl Glittergold and Flandal Steelskin. Keyed to gnomes, the only ones allowed to use this gateway are gnomes themselves, or those that prove themselves of good character (as judged by Bytopian standards, of course).
- Less well-known is the Smoke Ring, a construct of solid iron in the realm of Moradin in Mount Celestia built by the god himself. This gate opens only at the willing touch of Moradin himself, or one of his priests or proxies, and thus getting entry through it can be even more difficult than the Bytopian gate.
((I found this link and found it helpful brain-fodder: www.rilmani.org/timaresh/Paraelemental_Plane_of_Smoke ))

By limiting the passage of his competitors, Master of Cinders has a monopoly on certain rare ingredients only harvestable from the Plane of Smoke and seeks to transform this monopoly into great personal wealth and power.

When the PC is transported, perhaps they encounter Master of Cinders himself, but they could also just encounter a powerful native denizen of the plane as well, such as an eldar Belker