Malice are deadly viral organisms that have been created when the Calamity Ganon (actually Ganondorf gone insane) laid waste over the land of Hyrule. They made their debut in The Legend of Zelda: Breath of the Wild.
Malice behave similarly to how Dark Matter from the Super Mario Galaxy franchise behave, except Malice do not kill their target instantly. Their colonies (which can be considered hazards) vary in their shape, but all of them resemble a mass of purple stuff that has a network of crisscrossing red and white lines. A creature that is exposed to this viral life form will mutate into a more terrifying, more dangerous form. Malice can also reanimate corpses in their mass, creating Malice Zombies. These Zombies very much resemble their normal form, but are basically floating heads and attack like the Bubbles from the Zelda series do.
Despite their nature as hazards, Malice can be destroyed. Their patches are controlled by sentient glowing eyeballs. These eyeballs sometimes launch some Malice Zombies from their respective mouths, but destroying the eyeballs kills off any Malice controlled by them.
The Calamity Ganon himself appears as a giant, sentient Malice hunk, though he has some armaments on his body and some of his old self's features, such as a vaguely humanoid head, but his other features are also Lovecraftian: his body is covered with creepy crawly legs and his eyes glow a menacing orange, their glow giving the monster a terrifying reputation.