The only evidence we have about their existence is that they are something of the Vasari's own making, have been chasing them for the past 10,000 years, drive the Vasari that see them insane, destroyed the Vasari Empire, what is on the Kultorask's underside (provided this is actually something and not just us seeing what we want to see), and that there is some other clue hidden somewhere in the .exe which apparently should only take about 7 minutes to find.
Anyways, the pic of the Kultorask's underside is this:
This can be interpreted a couple different ways that I can see. The first is one that portrays a Vasari (white) and a chaser (blue) in which the Vasari appears to be alarmed as the chaser attempts to eat it.
The second option has a Vasari head (white) with the chaser (blue) where it appears that the chaser is some sort of infection that walks around in a decapitated Vasari head. Alternatively, this could just be an image of a Vasari face (my personal view) and the "chaser" is just the lines that happen to be nearby to make the ship look cool.
When compared to the original Vasari head, it IMO makes this option more likely simply as it's a better resemblance.