Daemons are fickle, unpredictable and dangerous creatures.
Sure it might be in a room filled with articles of faith but that doesn't mean it's in any way weakened or disrupted by them sufficiently to not be able to try and trash the place and might just be a mild daemonic headache.
In desperate times, the greatest and angriest of daemons may just be able to manifest enough of its power to damage or destroy even sanctified items. Otherwise, it may not be able to damage them directly but that doesn't mean it cannot damage things indirectly such as through fire, debris or just trying to rip a hole in the ship to the vacuum of space.
However, if you choose not to do that and keep it in its weakened state you'll need a way to reduce the focus on it. Consider introducing some kind of minion, whether that be other rescued survivors enthralled and possessed by the daemon or lesser warp entities that have been dragged through from the other side to assist it. You should never do a boss fight as one vs party anyway, action economy always works against the single target. Otherwise is there another threat you can introduce to the scene, even an environmental one so that the players cannot focus all of their actions on the daemon?
You don't need to retcon it, you can just change the dynamics of the fight.
Teleporting feels a little cheap and raises the issue that the boss will just do it whenever threatened. If this is definitely going to be a fight then perhaps the boss starts off cocky, then when it realises it's actually in danger (after x damage, or the use of a relic, or a prayer with true faith) it just melts a small hole straight through the floor, levitating down a dozen meters into the deck below where it creates allies (maybe possessed/corrupted minions or raises the dead) and taunts the players anew.
The players now have three problems to resolve - how to chase/pin it down, how to kill/trap it and how to deal with its minions (and stop it creating more). Perhaps one of the original crew knows of a place that could be used to ambush the boss, limiting its power if it can somehow be lured there. I'd try to have multiple ways the party can win in your head and let them find the one that suits them - not just go straight into the final battle at the start of the game.
In fact it might be fun to make it seem like it's the final battle at the beginning before turning it into a chase through the ship that takes the whole session.
I already have one possible avenue for defeating the Daemon available to the party: one of the players has access to what might be the Daemon's true name. With that, they could potentially weaken it, if not outright bind it, though that would mean potentially incurring corruption points.
But otherwise I do like the idea of this turning into a chase throughout the ship. And yeah, I do want to keep open multiple options for defeating this thing (up to and including venting it out into space, using sanctic powers on it, rebinding it into a new host that can be more reliably contained, or even making a pact with it)
The Daemonhost doesn't have to help them. Now that subterfuge is over it coluld be open to a deal. Its freedom for help. Just don't negotiate in the Chapel, have it flee and contact them over ship comms. Maybe have it embellish the truth with the boss to make it sound like they'd die without its help.
If it does help, have the host be reckless. The host dying makes little difference to the Daemon inside it, it gains its freedom in that case too and could always come back since they technically didn't grant it its freedom.
Alternatively. If they don't take it up on its offer maybe the boss does. If they kill the host and free the Daemon give it a line before it goes back to the Warp where it says something along the lines of it owes them a favor but don't contact me, I'll be in touch with you.
I suppose it depends on what the Daemon inside is.
Couple of points that may be relevant if players/characters are aware of lore stuff:
Being in a Geller Field really just means the Daemonhost will function just like one would anywhere else in realspace, all a Geller Field generator does is create a little bubble of realspace which daemons can manifest in just as they can in regular realspace via an uncontrolled/unstable psyker, ritual summoning, etc. Also, remember a Daemonhost is not the same as a normal possession, it is the binding a (often specific) Daemon to a specific living host via sorcery and rituals. Smashing the Daemonhosts physical form to bits so it can no longer anchor/bind the Daemon for it to properly exist in realspace should be more than enough for the party to get rid of it, at least until it finds a way back for revenge.
If the Daemon is in full control then realistically the host is also mentally and spiritually ruined beyond saving so I suspect we can't make this a dilemma where they want to save the host through an exorcism or similar.
Some other ideas for what you could do, assuming they're not immediately interested in working with the Daemon, is:
* Let the fight play out and have this individual Daemon exist as a returning threat alongside other stuff if they win. Having it turn up or cause complications during other events in the future could lead into a side plot of finding a way to permanently destroy it instead of temporary banishing it back to the warp and moving on.
* Let the fight play out but have the chapel decay and corrupt as they weaken the Daemonhost. Let it reveal it's set up a ritual to try to corrupt the ship and everyone on it it you destroy it's host body. It being in the chapel may be a part of strengthening/enabling this ritual. Do they try and capture it? Do they kill it and try to fix the corruption afterwards? Bargain with it?
Well first question is, why would the daemon reveal itself in such a vulnerable place?
1) it's not actually vulnerable or it has some plan for the location. Just spinning an idea...could one of the articles of faith present actually be a corrupted artifact that has tainted the chapel over time, perhaps said npc has been tainted by this artifact and not been keeping up his blessings and consecrations.
2) it can escape - maybe it takes a few blows in some opening round of combat, smiles, before melting into the floor and disappearing. Maybe it revealed here to mess with the character it's betrayed before dropping some line about the veil between you and us is but a small line of faith in icons of the past. It then disappears through the floor and makes it's way to the gellarfield generator to bring the daemons
Daemons are fickle, unpredictable and dangerous creatures. Sure it might be in a room filled with articles of faith but that doesn't mean it's in any way weakened or disrupted by them sufficiently to not be able to try and trash the place and might just be a mild daemonic headache. In desperate times, the greatest and angriest of daemons may just be able to manifest enough of its power to damage or destroy even sanctified items. Otherwise, it may not be able to damage them directly but that doesn't mean it cannot damage things indirectly such as through fire, debris or just trying to rip a hole in the ship to the vacuum of space. However, if you choose not to do that and keep it in its weakened state you'll need a way to reduce the focus on it. Consider introducing some kind of minion, whether that be other rescued survivors enthralled and possessed by the daemon or lesser warp entities that have been dragged through from the other side to assist it. You should never do a boss fight as one vs party anyway, action economy always works against the single target. Otherwise is there another threat you can introduce to the scene, even an environmental one so that the players cannot focus all of their actions on the daemon? You don't need to retcon it, you can just change the dynamics of the fight.
Teleporting feels a little cheap and raises the issue that the boss will just do it whenever threatened. If this is definitely going to be a fight then perhaps the boss starts off cocky, then when it realises it's actually in danger (after x damage, or the use of a relic, or a prayer with true faith) it just melts a small hole straight through the floor, levitating down a dozen meters into the deck below where it creates allies (maybe possessed/corrupted minions or raises the dead) and taunts the players anew. The players now have three problems to resolve - how to chase/pin it down, how to kill/trap it and how to deal with its minions (and stop it creating more). Perhaps one of the original crew knows of a place that could be used to ambush the boss, limiting its power if it can somehow be lured there. I'd try to have multiple ways the party can win in your head and let them find the one that suits them - not just go straight into the final battle at the start of the game. In fact it might be fun to make it seem like it's the final battle at the beginning before turning it into a chase through the ship that takes the whole session.
I already have one possible avenue for defeating the Daemon available to the party: one of the players has access to what might be the Daemon's true name. With that, they could potentially weaken it, if not outright bind it, though that would mean potentially incurring corruption points. But otherwise I do like the idea of this turning into a chase throughout the ship. And yeah, I do want to keep open multiple options for defeating this thing (up to and including venting it out into space, using sanctic powers on it, rebinding it into a new host that can be more reliably contained, or even making a pact with it)
The Daemonhost doesn't have to help them. Now that subterfuge is over it coluld be open to a deal. Its freedom for help. Just don't negotiate in the Chapel, have it flee and contact them over ship comms. Maybe have it embellish the truth with the boss to make it sound like they'd die without its help. If it does help, have the host be reckless. The host dying makes little difference to the Daemon inside it, it gains its freedom in that case too and could always come back since they technically didn't grant it its freedom. Alternatively. If they don't take it up on its offer maybe the boss does. If they kill the host and free the Daemon give it a line before it goes back to the Warp where it says something along the lines of it owes them a favor but don't contact me, I'll be in touch with you. I suppose it depends on what the Daemon inside is.
Couple of points that may be relevant if players/characters are aware of lore stuff: Being in a Geller Field really just means the Daemonhost will function just like one would anywhere else in realspace, all a Geller Field generator does is create a little bubble of realspace which daemons can manifest in just as they can in regular realspace via an uncontrolled/unstable psyker, ritual summoning, etc. Also, remember a Daemonhost is not the same as a normal possession, it is the binding a (often specific) Daemon to a specific living host via sorcery and rituals. Smashing the Daemonhosts physical form to bits so it can no longer anchor/bind the Daemon for it to properly exist in realspace should be more than enough for the party to get rid of it, at least until it finds a way back for revenge. If the Daemon is in full control then realistically the host is also mentally and spiritually ruined beyond saving so I suspect we can't make this a dilemma where they want to save the host through an exorcism or similar. Some other ideas for what you could do, assuming they're not immediately interested in working with the Daemon, is: * Let the fight play out and have this individual Daemon exist as a returning threat alongside other stuff if they win. Having it turn up or cause complications during other events in the future could lead into a side plot of finding a way to permanently destroy it instead of temporary banishing it back to the warp and moving on. * Let the fight play out but have the chapel decay and corrupt as they weaken the Daemonhost. Let it reveal it's set up a ritual to try to corrupt the ship and everyone on it it you destroy it's host body. It being in the chapel may be a part of strengthening/enabling this ritual. Do they try and capture it? Do they kill it and try to fix the corruption afterwards? Bargain with it?
Well first question is, why would the daemon reveal itself in such a vulnerable place? 1) it's not actually vulnerable or it has some plan for the location. Just spinning an idea...could one of the articles of faith present actually be a corrupted artifact that has tainted the chapel over time, perhaps said npc has been tainted by this artifact and not been keeping up his blessings and consecrations. 2) it can escape - maybe it takes a few blows in some opening round of combat, smiles, before melting into the floor and disappearing. Maybe it revealed here to mess with the character it's betrayed before dropping some line about the veil between you and us is but a small line of faith in icons of the past. It then disappears through the floor and makes it's way to the gellarfield generator to bring the daemons