From high level to low level, there are many different creatures you can throw at a party and many different ways to do so. For a while now I've wanted to go through a few different monsters and throw around some ideas for their use. It's an exercise that has helped me before in the past and I hope that people who find this get some use out of it as well. And given my long standing tradition of sending hordes of undead for my players to kill, where better to start than with the evil old lich?
These guys are amazingly versatile. From big bads, to interesting enemies in a dungeon to a cool thing for a low level party (this is atypical but I'll get back to this), there is a lot you can do with these guys. They also hit on a few themes that I for one find cool. Life, death, undeath, all the cool stuff. The phylactery is a quest just waiting to happen as well. Since just killing them isn't enough either, and liches typically don't carry their insurance policy on them, chances are high that players will meet the same one more than once before finally putting an end to them.
Playing With Alignment
These guys are evil. Right? Well, playing around with their alignments and goals can yield a lot of interesting situations. One of the first campaigns I played in had a lich that had been transformed against their will and tried whatever they could do to regain control of their own unlife. They couldn't escape undeath as long as their phylactery was held captive and they couldn't rebel without consequences either.
While these creatures tend to be portrayed as evil, all of the other alignments present opportunities for something new. It largely goes back to what your world thinks about trying to dodge the regular flow of life, but I've seen interpretations before where they were the main ally for the players. Now, of course, they had their own interests and their undead nature fed into their character and how they saw the world. However, they did try to help the party as well and wanted to leave the world in a better state. One I remember very fondly, though I don't recall if I've mentioned him before in my writing, was a brilliant inventor and mage that would massage events in order to have his inventions fall into the right hands to spread and improve the world. The only issue is that things rarely go as planned, and the inventions would often cause much destruction as well.
Liches For Low Level Players
Low level players also want cool things to fight and situations to encounter. Whether you are level 1 or 20, undead can still be cool and tough. For level 20, the classic lich is perfectly fine. For low level players, a rotting, falling apart lich who stays undead through pure sheer of will after being starved without their phylactery is a lot of fun. Now, I'd be careful about throwing one of these at your players without them knowing what happened here. Otherwise the next time they meet a lich, it will be an absolute disaster.
They tend to have tons of undead minions, and possibly cults working in their name. They spin plans that span hundreds of years thanks to their lack of permanent death. They are one of the first things that comes to my mind when someone says “big bad”. That said, we can have a little more fun with these guys than that. Since anything can die and become undead, we can have a lot of fun with undead armies. Undead minotaurs, skeletons, ogres, and dragons all rolling in and attacking a city. Being that they are spellcasters as well, you can do amazing things by just tweaking their spell lists. Having them as combat challenges for lower level ranges can also work, but you'll need some justification for it. The methods that come to mind are having their spellbook destroyed, reducing their spell list, a curse preventing or injury preventing them from regaining spell slots, or simply never being strong enough to complete the process themselves and instead having been turned by their more gifted master (their magic skills reflect this).
What Happens After?
D&D and the campaigns I've been involved in often have legends of liches that became for greater than mere liches. From gods to forces of nature that project their strength from the negative energy plane, the goals of a lich and what they achieve can be vast. Don't feel you need to stop at just a lich.
Since we are dealing with a spellcaster, remember that playing with the spell list can greatly change everything. You can make a defense minded lich, a pyromancer, a seer that manipulates events for centuries, and many others. It should, of course, reflect their character but this versatility helps keep them from becoming boring.
Another one of my fond memories was a conflict hundreds of years long between two liches. They bested each other and destroyed their bodies time and time again, savagely unravel each-other's plans both for reasons of gain and spite, but had never been able to find each-other's phylactery to finally end it. It's was an entertaining thing to witness and also has a lot of potential as the setup for a campaign. It lends itself quite well to games of intrigue as well.