Negative energy pollution. It's basically anti-life energy, which is really bad for the environment. It does shit like make bird-shells too thin to be viable, bleaches coral reefs, blackens soil, kills trees, and starves cute endangered bears. Basically it's why necromancer realms all look like shit-holes.
Also it torments the souls of the dead. That can include you, if a necromancer gets his paws on your remains.