TPK? This is when the Explorers start being offered demonic pacts by certain Chaos forces to keep the other Chaos forces out, resulting in one super-powerful Demon forcing your PCs to do some ridiculously dangerous plot or sacrifice their souls and the souls of everyone on board! Or the Gellar Field goes to "reduced status" or you pre-emptively vent the atmosphere killing the majority of your crew but lowering the psychic emanation of your vessel to hopefully pass undetected!

Anyway, once the ship returns to Realspace presumably the weaker warp entities would fade away, while the stronger ones would gradually succumb to Warp Instability.

Erathia said:

That is an alternative I had considered (Packs with Demons) however knowing the righteousness of my explorers, I bet they would rather die than make a deal - It will be a good option to give them though

If some of them would notice the plasma drive going down, I think a skilled Navigator can pull you into realspace on backup power, so no harm done there. However, if they can't, their Missionary better prepare and sanctify the bridge (or wherever they take refuge). That could keep away some minor warp threats, and maybe weaken the stronger ones. They'll fight, and the transition back to realspace would outright destroy most minor entities, as written in previous posts.

Otherwise, the posts above have good ideas, I can't think of any more at the moment.

Anything!

Anything can happend when the Geller Field breaks down, the warp follows few real laws, it is possible the ship would be ripped to pieces before any deamon turns up, maybe every living person on the ship turns to sand the instant it is fully unprotected, maybe a daemonic incursion breaks lose, maybe large brass skulls start to bombard the ship. There are ships that drift out of the warp after such malfunctions at times and while those tend to be dangerous the exact way varies greatly.

Just decide what effect you want it to have on the explorers and come up for a way for it to have that effect.

If there is still reserve power keeping the geller field running but at decreased capacity the ship still has protection in a form and any happening due to the effects should be possible but hard to survive.

If you believe The Navis Primer, a failed gellar field is not that big a deal (though still bad). The journey may become longer (presumably because the navigator has to avoid areas in the warp that are dangerous without a gellar field), and some Warp Travel Encounters and Warp Incursions become far worse.

I have another idea on this matter. As was pointed out by Iku Rex, there is the potentially that the ship transitions back in with very little damage. However this should also have the potential to completely devastate your ship was Warp Predators fall in. Thus I propose a three-step method to damning your players if the Gellar Field collapses.

1) Roll d100. Their ship loses that result in Crew Population and Morale. As the Warp is chaotic and unpredictable, this could leave the Explorers suddenly with a nigh-ghost ship on board, or relatively unscathed. If you don't want to completely screw them over, then if you roll above 50, take 100-your result as the amount that was lost so they will only suffer at worst a 50% crew depletion.

2) Force the Explorers to make a Hard (-20) Command Test or the Ship's surviving morale is halved at the result of their close encounter with the Warp. Other characters may make assist tests such as Charm, Intimidate, Tech-Use to reassure that the ship is functional to improve the difficulty of this test.

3) There is now a Warp Predator lurking abord the ship. It is going through Components killing crew members as it passes. Select a random Component and treat it as damaged. This continues until the Explorers confront and destroy the Predator, or 1d5 Components are attacked at which point it phases back into the Warp.

Yeah, I would think that decent portions of your ship's crew would actually be all right; I don't want to give spoilers, but one of the stopping points in Lure of the Expanse is the hulk of the Light of Terra, a battleship from the Angevin Crusade that got lost in the warp for ages. Their gellar field did fail, and they certainly had problems, with a Daemon possessing a random crewer, and committing murders across the decks, and eventually the strain of the warp did wipe out all of the Navigators, who killed themselves (I assume the Astropaths probably emulated them). This ship was lost in the warp, AND trapped in a storm for over 1,000 years, and still eventually out of the warp with portions of their benighted crew, who went on to found the Tribes of Terra aboard the ship.

So, problems can occur, but warp incursions don't often seem to run like Ork raids or Nid Rushs; the warp is a great, vast space, filled with lots of nothing, and lots of mindless beasts. You probably won't have packs of Screamers heralding Bloodletters up the hallways, while Nurglings cackle from recesses behind your walls. Then, hopefully your Navigator will transition you back quick, or your Explorator will get to work on those repairs. Servitors are practically immune to warp incursions, so if the Tech-Priest and his/her drones hole up in the Enginarium, they should be able to get things working well enough to regain power, at least long enough to dump the ship back out to realspace. Once there, the lesser daemons, without the warp power, or rituals keeping them around will probably fade away, while the bigger baddies will do the same, or slowly get killed in battle. Aboard the Light, there is not a single daemon encounter, so I'd guess as time passes, hosts age or die, and lasblasts ring out, even the tenacious possessor daemons will get nailed.

The true nasty threat might be what happens AFTER the ship gets back. Does the Inquisition find out about the incident? Do they care? (They are often bored, and just looking for cases to nose around in.) How bad off is the ship? There could be BIG things in the warp area you were in, like their equivalent of a Void Kraken, or a ship of Chaos Raiders, either of which could do a number on your hull. Your plasma drive could be kaput. Once you are back out, there are plenty of raiders, pirates, and problems waiting for a limping ship to hobble on by, possibly more than in the warp. I hope your group has fun with all of what you choose; the gellar field fail, regardless of its seriousness mechanically, is one of the single worst nightmares most spacers can imagine.

venkelos said:

(There was "a rent in the Gellar Field", which let in an "aberrant spectre", but I don't think it was a complete system failure.)

All GW fiction i have ever read has 2 outcomes for gellar field failure:
1. Mortal beings go 100% insane from exposure to the warp, often being mutated as time goes on. The ship will be swamped with minor warp entities and any bigger warp entitites that have been paying attention. If the vessel survives it will be without crew and possibly permanently possessed.

2. Astartes have been known to be able to view the warp without protection, but these are virtually always chaos space marines so it can be assumed that pacts and favours with daemons are protecting them from the worst effects.

Generally speaking a complete Gellar field failure should result in instant and ongoing Insanity gain - 1d10pts per round - and corruption gain depending on how directly exposed the PCs are. Simply gazing upon the warp might only generate 1 or 2 d10 corruption, but direct contact with a warp incursion and daemons would result in at least double that.

As has been said however, a ship is not 100% powered by its plasma drive and so backup would enable a skilled Navigator to pull the ship from the warp in an emergency - however it will be dangerous to the ship and likely leave you off course and possibly thrown through time also.

One of the published campaign books for Deathwatch (I won't name which one because it would be a spoiler) features a complete breakdown of the ship's gellar field and all this ensues. In that situation it results in an almost immediate invasion by uncountable numbers of demons. They become so numerous that they climb on top of each other and almost form a demonic wall that devours anything in its path. Suffice to say, anything living in the ship is dead within minutes.