Out of curiosity- what IS the level of challenge likely to be (with what the players have done) assuming you don't buff it? Is it going to be an "easy" encounter now, or still medium or hard (or even a slightly less deadly)?
Would it have been a "deadly" encounter without all that they did to mitigate?
Just curious and looking for a fuller picture.
If they had just gone into this without any planning, it would have been a multiple stage boss fight that they would have gone into fully rested. The first stage was around Deadly by the encounter math, which for this party fully rested ikely means someone may or may not hit unconcious, but nowhere near an actual death. Though it would be complicated by a bunch of low challenge guards (CR1 vs. 8th level party) who are basically innocent and protecting the duke from an assassination attempt by known fugitives - the party is sworn the the rightful ruler and (to varying degress) will not go murderhobo on non-evil citizens trying to do their job. The second stage would have also been Deadly+, as unexpectedly the Oinoloth comes out of the body of the Duke, with some minions.
Just for calibration, when fully rested it takes a Deadly encounter to make it interesting for this party. Not super optimized and a bit combat-magic-item-light (by my intention), but just solid tactics, teamwork and synergy.
They now know about the Oinoloth, and have researched how to expel it from the Duke's body and come up with a
most excellent plan on how to do it reliably and quickly that is so genius I must not DM fiat it away in any way. That will take a few rounds, but they can be on the defensive for them. And it will reveal the Oinoloth which will get all of the innocent guards out of the way.
This puts it down to a single Deadly+ encounter, plus they researched the Oinoloth and
know what it can do and what it is resistant to (perhaps the biggest swing in power
), managed to get a loan of additional magical weapons so that much of the party wouldn't be having their attacks Resisted, are controlling the time so they can blow major consumables like a potion of Frost Giant strength, and have tipped off other forces like the clergy to be on alert in such a way that shouldn't get back to him negatively.
Also they were very worried about the duke supposed oracle/diviner, but have found out that he's got his own agenda and wont' be stepping in to fight, so they don't need to spend effort to counter him.
There's still one twist in there that can get them, and if it was just a fresh party a Deadly+ would be a good climax with chance of people down at several points (though they have a twilight cleric, so not as much). But being at full, knowing the abilities of the foe, having prepped spells (cleric & druid) specifically to deal with it in this situation, borrowed magic items (weapons and consumables), being pre-buffed, and coming in from ambush. That's a pretty huge swing even before potential aid like the bishop of the the town.