Hello everyone! I'm working on making an end of arc boss fight for 7 level 11 players.  The boss is a formal ally who is falling to their demonic nature, but fighting every step of the way. Do you have any ideas on how to actualize this in combat? I think I'm going to have a percentile die that I roll each turn to see if she is able to resist her impulses, but beyond that I'm a bit stumped. 

What do you all think?

I think rolling Constitution (or other relevant stat, like Wisdom) saving throws would be a true-to-character way to play that, mostly like you’ve already figured out. 

Maybe let the characters decide—maybe up to one character a round—whether to try to sway/inspire her and let them give her advantage on saves, or take their full turn. How much of a choice that is depends on the risk, how much damage she can do how quickly if they don’t hurry up and deal with this. 

Demonic transformation:

• Wisdom save DC 13 at the beginning of every turn.
• On a successful save the Orc is able act as normal but movement is halved.
• On a failed save the Orc rushes the nearest creature and makes an attack against it. It uses the weapon it is currently holding, if unarmed it makes an unarmed attack.
• On a failed save the DC increases by 1
• On 3 failed saves the demonic power consumes the Orc and will consider all creatures an enemy.

Use some narrative flare to explain a black aura surrounding the orc after the first failed save. Describe the physical changes, glowing eyes, horns growing, claws appearing on the hand, etc. As well, make sure the Orc speaks, calls out for them to run, apologizes for attacks, as well, the demonic side bragging about winning this body over.

Also consider what spells may slow down, or help the Orc resist. Combat could end without a single attack roll made by the players if they're able to help stop the demonic entity.

Is combat the best way to work the progression?  I would think that you could also introduce odd character traits or unusual events that happen when one or more of the characters was in his presence.  Then by the time you are ready for the showdown, you begin with a normal meeting... with more frequent unusual behavior before devolving into the level of demonic rage necessary to provoke them into attacking.  Personally, I wouldn't use dice for any of it until you are ready for the players to roll initiative.  It is a good point if you want to have the characters be able to help and use their abilities, though you could also have them using their ability checks to try and figure out what is going on if the ending is predetermined.

My suggestion is to have a two enemies your party faces, one is the demonic presence that is actively trying to take over the former friend the other is that friend.

Rather than the former friend being slowly being taken over during the fight I would go the opposite route.  He begins the fight completely in the thrall of this demonic presence attacking in tandem with the spirit of the demon.  As the fight progresses the players can make entreaties to attempt to bring him back to his senses with the chance of that succeeding starting fairly low and becoming easier as the demonic presence loses hitpoints.  You could also have it be a bit of the balancing scales and as you deal damage to the former friend they become more in the thrall of the demon side, more enraged by the pain.  

I think something along the lines of the starting DC of talking your friend back to their former self starting at DC 22 when the demon is at full health and going down by 1 for every 15% health that is lost by the manifestation.  Check against it would be persuasion, but I feel like you could allow other checks that might be relevant such as reminding them of certain arcane principles if they were a mage, or intimidation to yell at them about the past they've shared/authoritative SNAP OUT OF IT, you aren't this weak kind of pep talk.  When these convince attempts succeed you can either have them do nothing for that round as they fight against the corruption and lower the DC of subsequent attempts, or do it more along the lines of the death saving throws where they have to get a certain number of successes before they have a certain number of failures.

I would say for every 20% of the health lost by the former friend the convince DC would back up by 1.  

This would result in a DC of 22 at the start (may want it higher with the level of our party) and a DC of 15 when the demonic presence is destroyed and no damage has been dealt to the former friend.  DC 21 when both enemies are at half health, DC 19 if the demon is gone and the former friend is at critical health.  These are difficult dcs most times but sounds like you have a lot of players that can make these attempts.  As a general side note I would suggest a trickle of other enemies coming into the combat area (minor demons or some such) to increase the sense of urgency the players feel.

This is not laid out in the most coherent manner but hopefully this gives you some ideas.

Thanks all! I think I have this figured out, your advice has been super helpful.

When you have all your details nailed down let us know how it played out.  I myself am fairly curious how it goes for everyone involved.

I hope they save the guy, but not easily.