Zina (fornication) is a major sin and the punishment of majors sins is hellfire if the sinners does not repent and sincerely seek forgiveness. Allah says in Quran: And come not near Zia, a transgression and an evil way. Sura Al-Israr', verse 32.
Zina is mentioned along with Shirk, and killing as most dangerous sins which must be avoided by the servants of Allah ( Sura Al-Forman, verse 68).
Wassalam.