The Bellows of Milil is actually part of Arbalest’s House. That is, its swellbox, windchest, most of its pipes, its stops, and the three rooms with lifting and falling (pumping) floors that form its air reservoir and bellows are all built into the walls, floors, and ceilings of the (largely stone) temple. The entire building resonates when the organ is played, leading to deeper notes that can be felt for blocks around, and even in the harbour waters. Many parts of the Bellows are large enough to walk in, and hide in, and there are temple rumours (some of them true) about priests hiding in side-chambers within the organ, some of which can be sealed off so an occupant isn’t killed by the sound reverberations pulverizing their brains if the organ is played while they’re inside. Other rumors (also true!) say that certain of the internal wind-passages and chambers of the organ are routes to gates/portals (upright midair ovals) created by Milil that lead to other temples, planes where the god dwells or has power or likes to visit, and locales in the wilderlands of the Realms where he likes to sing and play. Much of the temple’s treasure (in terms of perfectly-formed gemstones, not coins for everyday transactions) is hidden in false pipes amid the real ones, around the outside of the organ. The vast organ has three separate consoles that it can be played from: the main front seat with full pedals and four tiers of keyboards, a small “upper console” used by a second player to manage swells and hold drone-chords during some of the most sacred (critics sometimes use the word “fearsome”) anthems, and a hidden interior master console that can lock or silence (by disconnecting) the other consoles, and so silence the entire organ, as well as overriding the other consoles to play the organ. It is most often used to secure the organ for repairs, so priests (the only folk normally allowed to approach or touch the Bellows) can travel through it for cleaning, maintenance, and repairs.
An additional temple rumour, which has been true at some times, is that various “monsters” live within the Bellows, from wind walkers to volts. Tales of “The Wind Dragon” are entirely fanciful, but various creatures have entered the Bellows through gates and bided within for a time; certain senior priests believe that Milil himself sends some of them for his own purposes.
The Bellows has several spells built into it by Milil that are unleashed by playing particular chords (“wild” combinations that would never be played in most music-making). One of them creates a strong defensive magical ward around the temple, another unleashes a beam of magic that can be aimed to attack targets in the air, on the sea, or on the ground near the temple, and the third is some sort of mass teleport, details unknown.
Original thread on twitter: