I have some plausible and fun theories about Desmond Hart and his powers.

I suspect Shai-Hulud gifted him with familiar abilities. Or at least recognizable abilities, in the Dune universe. It doesn't have to be a boring technology that many of you (and I) have grimly anticipated.

Maybe Hart has an awareness spectrum that is unique. Sorta like Paul and Alia, but with some slightly novel additions:

The "Other Memories" within that Reverend Mother Kasha (and others) were accessible to Hart. But not just the female line. He can access the male memories. Thus was he able to influence it/them to do his will at the time of his choosing (in Kasha's case).

So he is able to interact with the genetic memory housed within certain other people in a meaningful way. His words, or even his face alone may carry esoteric messages to the “memory-presence.” It really depends on who these Memories are.

Hart instructs this memory-personality within the other person to alter their metabolism severely and catastrophically, and they begin to burn from within. With molecular control all sorts of crazy stuff can happen. And lots of fun ideas there. Keep in mind: we don't know who he is or who the Memory is.

Hart can access the male ancestor-memories, so the BG have no ability to detect what he's doing. The Other Memory within Kasha and the Richese men is shared by him. Or they share an understanding and a goal. Or they're in his power. Who knows? – he may be a relative or even an ancestor himself somehow.

It's hard to really pin down how this genetic memory-communication stuff works (for me anyway). So maybe Hart has some special resonance with Memory in others -- A mysterious type of communication, a la Alia. Direct communication of some sort, to some degree, was suggested as possible in the novels.

Thus Desmond Hart is an Abomination! “The Burning Truth” prophecy may be the unpredictable and chaotic nature of the Other Memory. It's inherently outside the BG's scope, and inevitably results in unintended consequences. A perfect Dune theme. The central theme, in fact (imo).

Lemme know if you all think this is possible or not