Lich

How to run it

The lich is a full 18th-level wizard with three legendary actions and it should never be in melee. Round one is usually a wall or a control spell, not damage. It has counterspell prepared and should hold a reaction for the party's first serious casting.

Disrupt Life costs three legendary actions for 6d6 necrotic to everyone within 20 feet, which punishes a party that clusters to heal. Paralyzing Touch at DC 18 removes a character for a minute, and the lich can use it as a legendary action too.

It is intelligent, patient and has prepared for this. A lich that fights fair is out of character. It should have wards, minions, an escape plan and a reason it can afford to lose.

Building the encounter

One lich is a deadly fight for a party of four at level 16 or 17, and it is the end of a campaign arc rather than an encounter. Immunity to nonmagical weapons alone will stop some parties dead.

The phylactery is the encounter. Rejuvenation means killing the lich accomplishes nothing permanent while the phylactery survives, so the real adventure is finding it. A lich fought without that groundwork is a very hard fight that changes nothing.

What to pair it with

Undead it raised, which animate dead supplies in fiction and in its prepared list. Skeletons and zombies as a screen while the lich casts from behind them is the classic arrangement and it works.

A second spellcaster is usually a mistake: two counterspell users makes the fight miserable rather than difficult. Bodies to absorb attacks are the better addition.

Reskins

An ancient intelligent caster who cannot be permanently killed. Reflavour as a bound archmage, a god's remnant, an AI in a crypt, or something that has been dying for a thousand years.

Rejuvenation is the trait worth keeping in any reskin, because it converts a boss fight into a campaign. Without it you have a hard encounter; with it you have a story.

Commonly played wrong

Turn Resistance gives advantage on saves against turning, not immunity. Clerics can still turn a lich, and letting that land occasionally is a good moment for the cleric player.

Paralyzing Touch is a melee spell attack, so it does not benefit from anything that boosts weapon attacks, and the lich has to be adjacent to use it. A lich reaching into melee to paralyse someone is taking a real risk and should have a reason.

Rejuvenation requires an intact phylactery, and the lich reappears within 5 feet of it in 1d10 days. That is a lot of time. Parties often assume it is instant and panic unnecessarily.

Compared to others at this CR

At CR 21 the lich is the control option among the highest-tier creatures. An adult red dragon at CR 17 does far more raw damage; the lich decides who is allowed to act.

What separates it from everything else in the SRD is that its stat block is not the encounter. A dragon's threat is entirely on the page. A lich's threat is the phylactery, the preparation and the fact that it is willing to lose today. Run it as a stat block and it is a slightly disappointing CR 21.