Malware
FantomCrypt
According to PCrisk, Fantom is a ransomware-type virus that imitates the Windows update procedure while encrypting files.
FantomCrypt is a Windows malware family.
Background
PCrisk describes Fantom as ransomware that disguises its file-encryption activity behind a fake Windows update screen. This behavior is atypical, as most ransomware encrypts quietly without any visible indication. While encrypting, Fantom adds the ".locked4", ".fantom" or ".locked" extension to the affected file names.
Source: Malpedia (Fraunhofer FKIE).