Fra Pelagatti's Lion, a/k/a the Castle-killing device, is a machine which creates a "low-pulse ætheric 'roar' punctuated by short bursts of intense trans-dimensional dissonance harmonics" The particular model we see in the comic is constructed by a group of Castle Heterodyne inmates at the instigation of Zola, using scrounged and smuggled-in parts; Professor Tiktoffen argues against its use, but is overruled.
From the description, it seems likely to function in a manner similar to an electromagnetic pulse, in which fluctuating fields interact with sensitive equipment to cause damaging current/voltage/energy surges. The "ætheric roar" provides a "static" which would disrupt local communications of a wireless or ætheric nature; the short bursts of dissonance would create the analogous waves which would provoke surges in reaction to the input energies.
After being apparently Agatha, it is revealed to have been preserved and possibly modified so that it shuts down the Castle temporarily rather than killing it outright. Agatha The Lion for this purpose when the Castle's ongoing mental deterioration leads it to attempt to prevent her from curing Gil, Tarvek and herself of Hogfarb's Resplendent Immolation.by
The Print Novels
The name of the device comes from the scientist who developed or best presented the functional principle(s) (see also: Occam's Razor, Schrödinger's Cat, Maxwell's Demon et alia). This is never discussed in the comic proper, but the third print novel reveals that Fra Pelagatti is/was an Abbot of the Corbettite Monks.
Possibly Relevant Outside Information
The title "Fra" refers to a friar or a monk. A similar title, "Fraa", receives extensive use in Neal Stephenson's recent novel, Anathem, which also deals with (mad) scientists sequestered in castles.