I like the concept of a lich but the one in the MM would mop the floor with all but the highest-level parties (and probably even them). Are there any similar antagonists that low or mid-level PCs have a better chance against?
Several undead creatures in the Monster Manual are roughly similar to liches. The wight (page 300) is a scary-looking dead person, albeit more focused on melee than spellcasting. The revenant (page 259) has the power to inhabit a new body if its current body is destroyed, and its Vengeful Glare attack resembles the lich's Frightening Gaze. The mummy and mummy lord (227-229) have Dreadful Glare, and the mummy lord's dependence on a canopic jar recalls the lich's phylactery.
You could use one of these creatures instead of a lich in your campaign; you could use the same stats but say that it's a lich; or you could modify a creature's statistics to more closely resemble those of the lich, ideally without increasing its difficulty. Those last two options are a little bit risky, because D&D's liches are famously difficult opponents for even high-level PCs, and a savvy player will be confused or maybe angry if you try to pass off a CR 3 wight as a lich. (You can also build a lower-CR monster from scratch, but that's not always easy to do, especially with something as complicated as a lich.)
The mummy lord seems like a good candidate for minor tweaking. It's balanced for a level 15 party (roughly) and, like a lich, it has a glare attack and spellcasting powers. The main difference is that the mummy lord's spells all come from the Cleric's spell list, where the lich's come from the Wizard's. Here are some changes I'd make:
- Swap out the mummy lord's spells for some of the lich's. (Leave the numbers for spell slots and spells known the same, though!)
- Switch the mummy lord's Intelligence and Wisdom scores, and change its spellcasting ability to Intelligence.
- Replace the mummy lord's damage vulnerability, resistances, and immunities with the lich's. The mummy lord is vulnerable to fire, and weakness might be important for balance, but it doesn't make a whole lot of sense for a lich. Maybe make it vulnerable to radiant?
- Change the Rotting Fist attack to the lich's Paralyzing Touch, but keep the mummy lord's to-hit modifier and save DCs.
- The Blinding Dust and Whirlwind of Sand powers are written to match a desert theme, so you'll want to change that flavor if your lich lives in a swamp or the North Pole.
It's very easy to "trick" your players by modifying a monster's aesthetic qualities. You can tell them a gorilla is an owlbear, or you can say an owlbear is a gorilla. You could convince them that a remorhaz is a freight train. As long as you don't change the numbers, you can trust the printed challenge rating as you build your encounter.
(Well, that is, you can trust the printed challenge rating as much as you could in the first place. I think you can probably trust the printed challenge rating. Don't worry about it.)
One of the old modules (don't recall the title, but it was before 2nd Ed. came out) had an undead enemy that the party were supposed to think was a lich; in fact, it was just an animated corpse with Magic Mouth to let it sit up in its sarcophagus and shout something (and get the party to waste whatever high-power strikes they might have obtained). You could probably do something similar with a collection of necromantic and other spells to create a walking corpse with some level of autonomy and possibly even intelligence -- but an overall power level little higher than that of a zombie.