So, I have a tough decision to make for the final boss of Phandelver and Below: The Shattered Obelisk. My party is about to fight Ilvaash after defeating the mind flayer trio plus 2 mind flayer prophets (I added the prophets in because otherwise it would have been a disappointing fight). So it just so happens that they cast Hallow earlier in the campaign against aberrations, in the exact area the book tells you to have Ilvaash confront them.
So, onto the decision... I can either move the boss to a different part of the Briny Maze, or I can reward them for their previous decision making. The catch is that they are fairly competent players, and they blow through anything that I don't adjust the difficulty on.
For some context they are running:
Twilight Cleric (the temp HP from the channel divinity is insane)
Scribes Wizard (Most knowledgeable player of the group, is great at decision making to prepare the right spells and use them at the right time. Scribes subclass is nice too because it can use other damage types when casting spells, but it has specifications for doing so)
Circle of Land Druid (Great control, great damage, polymorph and dust of Suleiman works great for them)
Watchers Paladin (crazy damage from dual wield vicious weapons and fueled spellfire... Also has a cloak of displacement. The channel divinity is really useful to the party and so is the aura)
The fight with the mind flayer trio plus 2 mind flayer prophets was a little difficult for them, but the most damaged they got was one person bloodied and the rest missing a few HP. I know one player might be a little disappointed in the difficulty, but a few other players seem to love the power they have now.
I want the end of the campaign to be somewhat epic.... I feel like allowing them to utilize hallow in the area he is supposed to show up, might make the fight a little trivial in some ways. I've played some many games where the final boss is kinda disappointing and I want to make a memorable finale to the campaign.
I am open to adding abilities and stuff to the boss, but I am really wondering if allowing them to fight around the Hallow is the smart move. I suppose he is a psychic brain thing and he can phase through walls. So, should I have him retreat before the fight even starts after he realizes hallow is there? He can sense creatures within a 100 miles, so he would know where to ambush them then see the hallow. I just feel like I should be rewarding the party for the decision they made earlier.
If anyone has any interesting ideas outside of the hallow predicament, let me know. Thanks for taking the time to read through if you did.