Basilisk weeds have been modified in two ways: first, to grow quickly and reproduce in great numbers, and second, to grow in ways that create basilisk hacks. Every aspect of their forms, from the veins in their leaves, the fractal patterns of their branches, the flow of their bark, and even the sound of wind through them, has been engineered to erode human minds. Basilisk hacks are precise things, however, and the basilisk weeds don't always grow in quite the right way to produce their full effect. Most often, basilisk weeds simply induce a feeling of vague dread, anxiety or confusion.
The majority of the basilisk weeds were, and hopefully are, on Earth. They would grow in terrible groves the locals learned to avoid, and those who wandered into forests would become confused, disoriented or catatonic, and never come back out.
Whenever a character views a basilisk weed, flip a coin. Heads, the character simply feels a sense of dread, tails they are effected by one of the incapacitating input type basilisk hacks.