Magic ink

 is formed when Kotatsu places a shard of the Shikon jewel in his ink jar. Although it looks normal at first sight, the ink needs a constant supply of human livers to sustain its life and would attack anyone if it is hungry. Moreover, anything painted with the ink comes to life. Also, because of it's mixture of blood and ink, it creates a scent strong enough to knock Inuyasha out.

Trivia
Strangely, Shippo, who has a strong sense of smell, is not affected by the scent of the Magic Ink. This may be because he was not covered in it, or that his sense was not fully developed at the time. Kirara is not affected either.