"Viola Bastarda" (potentially mislabled), made by Sanctus Seraphin, Venice, 1712
Catalog entry at the Musée de la musique in Paris
This is a roughly cello-sized instrument with 6 playable strings and 12 sympathetic strings, 18 in total. The catalog labels this instrument a "Viola Bastarda" which usually means a virtuosic and aggressive playstyle on a regular Viola da Gamba, not a type of instrument.
https://en.wikipedia.org/wiki/Viola_bastarda
Sound sample of the bastarda playing style on YouTube
The soundholes in the form of flaming swords, the rosette, the sympathetic strings and the blindfolded head on the pegbox suggest that this instrument is part of the "d'Amore" family, a Bass Viola d'Amore.
https://en.wikipedia.org/wiki/Viola_d%27amore
So far I haven't found another instrument like this, maybe it's unique?
damn i don't know how much you'd have to pay me to change strings on that thing, but it would be a lot
It's a real bastard to tune.
From the Wikipedia article you linked:
Oops! I might have overread that... Thanks for the correction!
Points for the weird, unsettling head
That's a sexy instrument