(Because I've read too much about the Derro in falsemachine's underdark writing, Veins and because I have tinnitus.)
The derro transmit their signal from somewhere near the pounding heart of the Urth. Certain surface-dwelling individuals receive it - typically those cretins who have bludgeoned their ears with musket fire or cannon or bomb. Their hearing is partly lost. Their brains quest for the missing sensation, like a man without an arm who still feels it itch. The transmission steps into their eager minds, in the form of a persistent ringing which does not fade. It warbles forever just at the edge of perception, softer or louder, always with one, always the same tone, a phantom itch in the mind. For now, the transmission is but a test pattern.
With this test pattern, the derro prevent sleep, or causes anxiety or depression, suicidal ideation or, horribly, the act itself. This naturally delights the derro.
But their goal is not to depress or promote self-destruction. Their goal is to build The Machine, to fuel it with blood, to calculate all of creation, anticipating every drop of rain, every frown, every betrayal. Their goal is omniscience over all things, usurping the place of the gods, removing from us our free will, such that it is.
For this they will need an army on the surface, to pull down nations and cast blood down into the cracks of the Urth. They will torture those who receive the signal and resist, simply by amplifying the test signal. To those who obey, they will broadcast orders, they will coordinate. An army will coalesce from a body of civilians, fight viciously, then fade back into their countryside or cityscape. They will have no leaders, no screamed orders or signal flags. Their disparate battalions will flock together and flank and attack and feint as though directed by a God, listening to invisible voices. Conventional armies will crumble.
But those so afflicted will find ways to rebel against their would-be tyrant. Shod in helms of lead, followed by shrill flutists, we will march into the deep wearing our Apostles, carrying cutlass and knife and musket. We will build the derro not a heaven of everything in its precise proper place, but a hellish cacophony. It will be our funeral song, chosen gladly, for it will be theirs as well.