Demontrumpets, as illuded to by the name, are not normal plants, but are plants who hold their origin in the Abyss. Perhaps brought by summoned demons, perhaps created by a dark god for some purpose, perhaps simple plants mutated by an artifact of great evil.
A large, ruby-red pepper, found in dry regions; much prized by alchemists and gourmands alike.
All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.