Your group will find the Final Boss Regret Demon bound inside a large mirror, capable of exerting influence but not of physically leaving or harming them (yet). Instead, it will show them their regrets, shifting between the appearances of people associated with those regrets, and try to convince them that it's powerful enough to help them go home (where applicable, particularly for rifters) and correct their mistakes or deliver them to worlds where they never happened.
Time travel is to some extent an established Thing in Thedas now, on top of the possibility of multiverse shenanigans, so this offer isn't completely outlandish. Additionally, the spirit will be meddling directly with their heads, amplifying the strength of those regrets and muddling their abilities to see reason, to try to make this sound more reasonable and more appealing. It will be strong enough that even very strongwilled people who would normally never agree to deal with a demon might need help resisting.
Fortunately, the demon will only be able to focus on one person at a time. (Whether it lingers on one person for several tags in a row or shifts constantly is up to you!) So the other people present can assist in countering its arguments and offers.
This mental influence will also be how the demon attempts to keep people away from the mirror or taking any direct action to attack or destroy it, with its focus shifting to people who try to approach. Dividing its attention will eventually allow someone to get close enough to smash the mirror.
Whether you would like that to defeat the demon or release it is up to you and whether you would like to have an actual boss fight instead of only a boss battle of wills. If you would prefer the fight, the demon will most closely resemble a stronger despair demon. If you wouldn't, destroying the mirror can send the demon directly back to the Fade because of Magic.
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Your group will find the Final Boss Regret Demon bound inside a large mirror, capable of exerting influence but not of physically leaving or harming them (yet). Instead, it will show them their regrets, shifting between the appearances of people associated with those regrets, and try to convince them that it's powerful enough to help them go home (where applicable, particularly for rifters) and correct their mistakes or deliver them to worlds where they never happened.
Time travel is to some extent an established Thing in Thedas now, on top of the possibility of multiverse shenanigans, so this offer isn't completely outlandish. Additionally, the spirit will be meddling directly with their heads, amplifying the strength of those regrets and muddling their abilities to see reason, to try to make this sound more reasonable and more appealing. It will be strong enough that even very strongwilled people who would normally never agree to deal with a demon might need help resisting.
Fortunately, the demon will only be able to focus on one person at a time. (Whether it lingers on one person for several tags in a row or shifts constantly is up to you!) So the other people present can assist in countering its arguments and offers.
This mental influence will also be how the demon attempts to keep people away from the mirror or taking any direct action to attack or destroy it, with its focus shifting to people who try to approach. Dividing its attention will eventually allow someone to get close enough to smash the mirror.
Whether you would like that to defeat the demon or release it is up to you and whether you would like to have an actual boss fight instead of only a boss battle of wills. If you would prefer the fight, the demon will most closely resemble a stronger despair demon. If you wouldn't, destroying the mirror can send the demon directly back to the Fade because of Magic.
Let me know if you have any questions!