What happens if an untouchable or a pariah enters the warp? Just curious. I couldn't find anything on this.

There is mention in the lore of extremely advanced scout vessels that are sent into the Eye of Terror to "map it out". Each is crewefby nothing but servitors and a single pariah.

Those are the only ships that can make it deep enough into the eye to send back information safely as the pariah inside effectively dampens the effects of the warp around him.

Going off of this information, we can assume that a pariah's abilities would extend into the Warp itself should they be exposed to it directly (such as in a Warp Incursion or Gellar Field failure aboard a starship). This should provide a modicum of portection, however the physical matter of their bodies would still suffer warping effects as the laws of nature fall away, not mutations mind you simply the matter itself becoming unstable and incoherent as the forces that bind it together become twisted to the point of nonexistence. Their mind would also break from seeing the raw energies of the Warp, as while they are immune to the corruption of the warp they are not immune to the insanity that comes with witnessing the Warp.

Another good reference point is the fact that Culexus assassins wear a suit that places them halfway between the Warp and reality, enough to render them out of sync with the rest of realspace but not enough to become crushed, squeezed and blown apart by the ravening energies of unreality within the Warp itself.

Exactly. An untouchable is not 'immune', merely resistant. Imagine them as someone who has an inverse* 'warp energy potential' compared to the rest of the universe, thus repelling people psychically and emotionally. The extent of this varies - they exist on the same scale as psykers, albeit at the other end, with the strongest mentioned being an omega-minus fetus stored in a stasis jar by the Inquisition, which does nasty things to Ahriman when the stasis shutter is opened.

But dropping even them in the warp without some sort of shielding is going to be the equivalent of rubbing an amber rod with a cloth and hoping this will somehow ward off a lightning bolt.

The Inquisitorial null-ships still have archaeotech Gellar fields and similar shielding - which are sufficient to make the experience just about survivable - for a pariah.

* Yes, all right, I know inverse potentials attract. It's a cruddy metaphor. Please don't poke the plot holes.

They have retconned that then. Pariahs and blanks used to be fully immune to warp powers not resistant.

Warp powers are not the same as nautral warp energies. The warp effects caused by psychic powers are created by a psyker manipulating the fabric of reality, as such Untouchables are immune to them seeing as their bodies are disconnected from the warp. This disconnect means that the energies summoned forth by the psyker find no purchase on the Untouchable.

However within the Warp itself, the energies cause the natural laws of reality to bend and twist in unnatural ways. Just as a pariah can still be killed by a rock thrown by a telekine, so do the ravening forces physically tear his body apart if within the warp.

They have retconned that then. Pariahs and blanks used to be fully immune to warp powers not resistant.

Even then, it dpended on the source. There where paria/null that were finally broken by very strong psychic energies and such. I just don't remeber the sources.

Exactly. One of the first black library appearances - in fact the first, now I think about it - of a Culexus Assassin was in the Inferno comic Daemonfuge.

As noted, it gets its head blown off by a psychic overdose trying to soul-drain Aphrael Stern.

Which, given her nature, was a pretty stupid idea all told.

Pariahs losing their immunity (or at least having it overridden by a sufficiently potent warp surge) have since appeared in the Ravenor series and the Horus Heresy series.

Never read the Ravenor books or that comic so my input on that subject is limited. My understanding of Pariahs was that their signature within the Warp was nonexistant making the use of Psychic Powers against them useless. However summoning forth pure Warp matter or having a Pariah be exposed to raw energies of the Immaterium could affect the matter their bodies are made of.

It was a matter of time and corrosion. The pariah in question was around an immensely powerful naucsent for an extended period of time that was having the psychic influence blocked by a daemon (if i remember right maybe it was that they thought it was a naucsent and it was just possession), so no one picked up on it. In the end the pariah's warp nullifying effect was simply beaten down over time and they eventually succumbed to its influence.