Arelith

This is a dungeon idea with four main components.

The first is a one-sentence summary of the dungeon.
The second is a narrative pitch.
The third is what the dungeon mechanically is.
The fourth is why I feel it might be a good idea.

Basic Summary :
Two brother liches protect their brother’s phylactery, so killing them is more difficult.

Narrative:
Two brothers of great arcane might find themselves allies throughout their entire lives. They were raised together, and scoured the land for any scrap of knowledge they could find.
Although both were gifted, neither could ever find a unique way to conquer their greatest enemy : time. Nor could they resist the temptations of lichdom.
These two decided then, to persist in undeath as they existed on the material plane. Together.
Even now, the two brothers toil in their shared tomb, where they both happily partake in the shared misery they inflict upon any who are confident enough to trespass into this unholy place.

Mechanical reasoning:
I don’t know about NWN’s engine limitations of how liches spawn in general. So I’m going to make a few assumptions here.
I’m going to assume that both the phylactery AND the lich must be on the same map.
I’m going to assume that both the phylactery and the lich need to be spawned at the same time as well, and must be within render distance of the player when it happens.

This means both liches, and phylacteries must be spawned at the same time.
Therefore, as soon as the party enters the “boss map” area, they should be able to see (but not inflict damage) upon the two brother-liches, and the phylacteries.

Since the party could see the two liches, it might be fun to have the two brothers throw in some taunts.
“Brother, look at how the fools are drawn in!”
“Want to bet on which of us kills more of them, brother?”
“One of us tells truths, the other tells lies… wait, we’re not doing that anymore this decade….”
“You look much weaker than the last group of fools we slew”
“Remember to share what you find off their corpses, brother!”
And so on.

Lich Brother 1 is near Phylactery 2, and just destroying the phylactery wouldn’t be enough to end the onslaught.
To stop the lich from following the party, they could use gates, slowing spells, or a locked door to try and slow the pursuit as they try to deal with the second phylactery on the other side of the last map.

The exact map layout could vary, and I don’t have a layout in mind.
I think it makes sense for the defeat of both Brothers to open up their treasury. Perhaps each of them drops a one-time-use key?

You can also make the two paths to each brother wildly different as a way of showing their two different mindsets. Maybe one favors traps, while the other favors swathes of the undead?
Maybe one is more sadistic, and tortures their prisoners?
Maybe one writes really bad poetry that the other brother can’t stand?
One brother really hates levers, and favors locked doors instead?

Why is this a good idea?

Liches tend to be placed too close to their phylacteries. I personally have felt that this makes some lich boss fights anti-climactic. I assume this is based off of engine limitations.
As a result, liches don’t feel as cunning as they could be.
I don’t think we need to have liches literally putting their phylactery in the Astral Plane, but this would be an improvement.

I think it also gives parties a real strategic challenge.
They could brute-force it if they’re strong enough but they may choose to split the party in the name of speed.
They could try and slow the liches through the halls, to give the team time to kill the other phylactery.
It could give a tank some time to shine to “hold him off”.

Roleplay opportunities! It’s begging for a comparison of the adventuring party’s relationships with their own siblings.