What has been your experience with the 12th-level lich straight from the Monster Core? I have seen 10th-level parties repeatedly lose to it as a moderate encounter.

In my various playtest runs, I have played as and GMed against 10th-level parties going up against a 12th-level lich as a moderate encounter. I have found that unless the party is specifically, expressly built to take down a lich, the PCs will almost certainly TPK: again, even as merely a moderate encounter.

Frightful Presence debuffs the party, first of all, and then come the spells. DC 36 is extreme for a 12th-level creature, leading to critical failures on saving throws, and failed counteract checks. Chain lightning can tear away tremendous chunks of Hit Points, dominate is very difficult to break out of, and Drain Soul Cage can restore either. Resist 10 cold is okay, but resist 10 physical (except magical bludgeoning) may force martials to bring out a backup weapon, and bow and crossbow specialist PCs might have no good backup weapon at all.

The difficulty spike between a lich and, say, a paleohemoth (another 12th-level rare from the exact same book) has been humongous.