Music by Kevin MacLeod. Available under the Creative Commons Attribution 3.0 Unported license. Download link: https://incompetech.com/music/royalty-free/index.html?isrc=USUAN1100802

MacLeod's description:

Genre: Soundtrack
Length: 0:49
Instruments: Percussion, Synths, Misc
Tempo: 53

Nice build-up and release. See also: Medusa
ISRC: US-UAN-11-00802
Dark, Epic, Intense, Suspenseful, Unnerving 2011