A thread of loose ends

[Aquillion]

[Speculation]
Xykon took the fake phylactery to his fortress. Any undead dependent upon a magic item for survival should be able to examine an item and know it is a fake, but even if he had to perform tests, he would still be able to determine if it is a fake or not. Why would he do so? Because he has never trusted Redcloak.

It would be an incredibly stupid act to hide a fake, spend months protecting it, and never realize what it is. But why didn't Xykon destroy RC when he found out? Because he made a replacement, hid it in his fortress, and now RC carries the fake. The strategic value in not confronting RC over the issue is, so long as RC thinks he has leverage, he is not actively working toward getting leverage.
[/Speculation]

While of course OOTS sometimes has its own rules, in 3.5e a lich cannot create a second phylactery under any circumstances. From Libre Mortis:

A lich can construct only a single phylactery. A lich whose phylactery is destroyed suffers no harm, but cannot construct a new one. If a lich without a phylactery is slain, the lich is forever destroyed.

It has his soul in it, he can't just arbitrarily decide to replace it with something new - certainly not without access to the phylactery itself.

Plus, we have a strong indicator that that's also how it works in OOTS. When Xykon's phylactery fell into the sewers he freaked out and spent massive amounts of time and effort retrieving it; if he could just create a new one, that wouldn't have been necessary.