Surbahar
The bass sitar, and the instrument that slows time down.
A larger body, longer scale and thicker strings tuned well below a sitar give the surbahar an enormous sustain and let you bend several notes on a single fret. That is why it belongs to the slow, deep alap of the dhrupad-influenced style, and why sitarists historically opened concerts on it before switching instruments.
Can a sitar player just pick one up? The layout is familiar but the string tension and left hand stretches are much heavier. Treat it as a related instrument to learn, not a large sitar.
What is played on it? Mostly alap, jor and jhala, the slow unfolding before rhythm enters.
Why are they rare? Small demand and demanding construction. Few makers build them, mostly to order, so expect us to confirm build time with you before any payment.