Hay new DM here and need some tips on how to RP this with my players.

Quick background, players closing portals to the Nine levels of Hell. Last fight three out of the six players got caught in a vortex and sent to Hell. So they're dead no coming back. Then our Cleric used divindivine intervention and rolled a Critical! So I allowed one of his party memebme to be reincarnated. 

So this character died in the 3rd level of Hell. You don't just come back from that unaffected. So im still deciding weather his soul is damaged or he didn't come back alone. But I'm more of a combat DM then a RP one. Any thoughts on what I can do?

I'd be a little wary of any sort of possession that could take away their agency completely. There's nothing worse for a player than being railroaded without any way of doing something about it.

A penalty would be a good idea. Perhaps something like: 'Stared into the abyss: During initiative, if an opposing creature is a demon, make a Wisdom saving throw (DC 10), or become frightened.'

Perhaps this character could confront their fears at some point in the future, to make peace with what they've seen. Could be a hook for an adventure!

A penalty would be a good idea. Perhaps something like: 'Stared into the abyss: During initiative, if an opposing creature is a demon, make a Wisdom saving throw (DC 10), or become frightened.

I really like this idea, I don't want to railrodr this character. I just want to add to the story and difficulty. I want it to be a challenge this guy has to face.

First off: did you read the Divine Intervention ability? There is no "critical", you roll d100 and have to get below the Cleric's level.

If the Divine Intervention ability replicated the spell True Resurrection, which makes sense given there was no body, it brings the creature back with essentially no ill effects. Given how unlikely Divine Intervention is to work, and how powerful it is intended to be (essentially almost a Wish), I'd be leery about doing mechanically bad things to this character. Just give him a new Flaw that reflects what little of the experience his mind can fathom - potentially look at the Madness section in the DMG.

I have a few ideas, but at the core they have these similarities: they should not infringe too much on gameplay that the player's choices and abilities are dampened, and that any penalty/change can be stopped or reversed with reasonable time and effort. You don't want you player thinking they'd have more fun with a new character than struggling along with a damazed, devil-infused heap.

1) Give the character a form of indefinite madness--Hell is a maddening place, you know?

2) Additionally/alternatively, it may be interesting to have the player take on some demonic traits as time passes...skin turning purple or eyes turning red, incisors starting to grow out...until they become a tiefling, the bloods of hell having infused with their own. Of course, I'd make sure there were plenty of chances for a player to stop or reverse any "permanent" change to their character. Something like Greater Restoration may seem simple enough, but the players still have to figure it out.

3) If you like the idea of possession, the character could become sort of a crying source--the demons are watching your party's movements through his eyes with divination magic. This, of course, should be able to be dispelled when/if the party realizes it.

I've been looking up the madness charts in the DMG, love it! I didn't know that was available to me. So we also have a druid who found a way back, for her I was thinking of ways how she will be affected. What do you think of this? Since she is a moon druid her transformation turn more demonic until she is able to remove this cures on her?

I think it would be cool RP flavor to havethe druid's beast shapes become more demonic day by day, without changing stats or mechanics. I don't know about how balanced it would be, but if she chooses not to do anything about it (over many sessions and in-game time) you could consider her Beast shape beasts as "fiends" instead of "beasts". Think Princess Mononoke style animals with black ooze dripping from them, etc. Once again, as long as it can be cured and it mostly RP fluff, it should work out well without frustrating your player

If the player died in the Nine Hells, it would be a devil rather than a demon. Also, look at the ghost (MM) and the dybbuk (MTOF) for ideas on possession. Otherwise your ideas seem good.

Yeah is it the Abyss or the Nine Hells? I guess it would be called a diabolical or devilish possession if it was the latter.

If it is the Nine Hells, consider that devils, being creatures of law and evil, are bound by myriad rules and restrictions and codes. Maybe the possession manifests itself in something the druid is forbidden from doing: killing a very specific type of creature, lying (or telling the truth), Wild Shaping into a creature with a flying speed, healing other creatures with spells, ect. ect.

Thank you for your additional contributions, however this thread was from 2018! One may consider that this Dungeon Master has now progressed past this point in the storyline. :) 

I will be locking this thread at this time. If this has sparked some new ideas for you that you'd like to discuss, we would love for you to make a new thread on this topic!

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Thank you!