I'm coming to the end of a long campaign and I certainly should have answered this question earlier...

Which 5e Demon is a good fit for this situation:

• The Demon has been trapped in a broken planar portal for many years.
• The Demon found someone (BBEG) about 6 months ago to manipulate to get it out of the portal.
• The party is unaware of the Demon, but they know the BBEG has bad intentions.
• Even the BBEG expects to either partner with or control the Demon, but this was a line fed to the BBEG by the Demon, pretending to be weaker than it is and that it would help the BBEG gain power.
• The BBEG has become a Warlock in a Pact with the Demon.

So the Demon needs to be:

• Intelligent or charismatic/coercive enough to have tricked the BBEG.
• A Demon that would WANT to get into the Material Plane and take over at least a portion of it. (Or is this just ALL Demons?)
• Powerful enough to be a Hard->Deadly challenge for a party of 5 well equipped Level 12 PCs, probably with allies.
• But NOT powerful enough to have been able to escape the broken portal on its own.

I've been looking at

• Balor (CR 19. But are they cunning enough to trick the BBEG?)
• Marilith
• Pit Fiend (Devil, not Demon, but maybe works better?)

Any thoughts or suggestions?

Quote from Cudder23 >>

I'm coming to the end of a long campaign and I certainly should have answered this question earlier...

Which 5e Demon is a good fit for this situation:

• The Demon has been trapped in a broken planar portal for many years.
• The Demon found someone (BBEG) about 6 months ago to manipulate to get it out of the portal.
• The party is unaware of the Demon, but they know the BBEG has bad intentions.
• Even the BBEG expects to either partner with or control the Demon, but this was a line fed to the BBEG by the Demon, pretending to be weaker than it is and that it would help the BBEG gain power.
• The BBEG has become a Warlock in a Pact with the Demon.

So the Demon needs to be:

• Intelligent or charismatic/coercive enough to have tricked the BBEG.
• A Demon that would WANT to get into the Material Plane and take over at least a portion of it. (Or is this just ALL Demons?)
• Powerful enough to be a Hard->Deadly challenge for a party of 5 well equipped Level 12 PCs, probably with allies.
• But NOT powerful enough to have been able to escape the broken portal on its own.

I've been looking at

• Balor (CR 19. But are they cunning enough to trick the BBEG?)
• Marilith
• Pit Fiend (Devil, not Demon, but maybe works better?)

Any thoughts or suggestions?

 Mephistopheles

downside to ^
5 level 12 pcs. It may be VERY deadly.

A broken portal can be an arbitrary challenge, though if you just call it the same as a Hedged Prison it will hold any generic demon, as none of them are level 17+ spellcasters. You can also round out the challenge by having multiple entities trapped instead of just one. In any case, all of those choices have plenty of Intelligence and Charisma, and you can always just create a custom variant (if it's a solo boss, I'd want legendary actions, though summoner variant demons that succeed on summoning are also hilariously deadly).

I think a Marilith is definitely cunning enough to do what you're looking for. You can always power up some other creature if you feel like it's a better narrative for and call it a demon. I think lorewise one of the layers of hell was ruled by a powerful night hag in an earlier edition. 

A Sibriex might be a good fit.

They are demonic lorekeepers that manipulate other demons to serve them, are quite strong (CR18), and very smart (+7 Int / Wis).

https://www.dndbeyond.com/monsters/sibriex

Oh. My. God. 

The Sibriex is outrageous:

• Feeblemind 3x/day!
• Warp Creature: DC 20 CON, poisoned and 1 level exhaustion on a fail... As an Action!
• Squirt Bile: DC 20 DEX or take 10d6 Acid!

That seems to me like it ought to be higher than DC 18...

But thank you very much for the suggestion!

I feel like it would probably mop the floor with them, but I may be underestimating the party. It's one relative weakness is that it only has 150 HP.

Thank you!

Honestly, my reservations with the Marilith is the feminine tone of the monster.

For one, the party fought a coven of hags, and I was wary of the untrustworthy female trope inherent in that monster. I tried to tone it down and portray the Hags as more monsters who happened to seem female rather than "evil women upending the rule of men" which is where I think the Hag/coven concept originates historically.

So I could use a Marilith and just recast it either male or androgynous. It does seem to have the right feel to it to me too.

Quote from Pantagruel666 >>

A broken portal can be an arbitrary challenge, though if you just call it the same as a Hedged Prison it will hold any generic demon, as none of them are level 17+ spellcasters. You can also round out the challenge by having multiple entities trapped instead of just one. In any case, all of those choices have plenty of Intelligence and Charisma, and you can always just create a custom variant (if it's a solo boss, I'd want legendary actions, though summoner variant demons that succeed on summoning are also hilariously deadly).

 This is awesome! Thank you! I know I can flavor or create mechanics for a "broken portal" anyway I want, but this suggestion of a Hedged Prison is awesome! I was not aware of the spell and it works perfectly for what I have in mind.