koban

1. fatal; fateful; disastrous
   To je bila kobna greška. — That was a fatal mistake.
   Te kobne noći sve se promenilo. — On that fateful night everything changed.
2. (of a sign, feeling) ominous; foreboding
   Nad gradom je vladala kobna tišina. — An ominous silence hung over the city.

Before a noun the long form is usual (kobni dan, kobna greška); as a predicate the short form: Taj dan je bio koban.