It is perfectly legal for anyone to go into the Forbidden Forest; it is just highly inadvisable to do so.
The Forbidden Forest is not a single specific part of the forests that make up much of Manta Carlos' biome. Rather, it is the collective name for those places far removed from civilization, where the high concentration of magic on the island coupled with decades of relative insulation from people have allowed the forest to grow into something wilder and, some would say, actively malevolent.
Sunlight is a rare feature in the Forbidden Forest, as the canopy is so thickly entangled within itself that it practically forms a ceiling. This has not stopped the underbrush from growing over immensely. Roughly a third of all plants in the Forbidden Forest are toxic in some way, and a not small minority are actively malicious (as in the case of the Chokevine and the Synthesis Flower).
The fauna is even more bizarre, and efforts to catalogue the beasts of the Forbidden Forest has been stagnated both by the high variation in the creatures (speculated to be due to rapid magical mutations) and the high mortality rate of those making the attempt.
