We show that the spectrum of the unusual transient SCP 06F6 is consistent with emission from a cool, optically thick and carbon-rich atmosphere if the transient is located at a redshift of z ≈ 0.14. The implied extragalactic nature of the transient rules out novae, shell flashes, and V838 Mon-like events as causes of the observed brightening. The distance to SCP 06F6 implies a peak magnitude of MI sime -18, in the regime of supernovae (SNe). While the morphology of the light curve of SCP 06F6 around the peak in brightness resembles the slowly evolving Type IIn supernovae SN 1994Y and SN 2006gy its spectroscopic appearance differs from all previous observed SNe. We further report the detection of an X-ray source coincident with SCP 06F6 in a target of opportunity XMM-Newton observation made during the declining phase of the transient. The X-ray luminosity of L X sime (5 ± 1) × 1042 erg s-1 is 2 orders of magnitude higher than observed to date from SNe. If related to an SN event, SCP 06F6 may define a new class. An alternative, though less likely, scenario is the tidal disruption of a carbon-rich star.