Few Monsters in the history of gaming evoke so much power and fear. The Lich is so much more than just a monster. When your DM introduces a Lich into your game your group is coming face to face with Legendary power. More often than not this will be a reoccurring major NPC in your game. Why you ask ? They are all major practitioners of magic possessing great skill, and limitless time to research, plot and scheme. They maintain all of the casting power that they had in life as well as now enjoying the benefits of undeath.. This makes them not only powerful but hard to kill begin with. You might succeed in hacking the body of a Lich to pieces and winning the day. But unless you know the location of his phylactery. The mystical device that houses the lich’s  soul, destroying its body will not kill it. Rather, its soul will return to the phylactery, and its body will be recreated by the power keeping it immortal.

The only way to permanently destroy a Lich is to destroy the phylactery. This is made more difficult due to a phylactery can be made of nearly anything or take any form.  With the phylactery being the most valued possession that a lich owns. It is almost always protected with powerful magic, traps and minions to defend it. Even more often a Lich will also place false phylactery’s about in hopes keep anyone in search guessing to its location.  If an adventuring group gets close enough to reach one of these false phylactery’s, He my even go into extended hiding. Either choosing to outlive his hunters or make them think that they have succeeded in their quest. In all thing’s a Lich’s driving goal is to survive. And when you can live forever. Sometimes it is easier to just hide and outlive your opposition.

If you do manage to kill a Lich and you have not destroyed its Phylactery you have at least bought you and your allies some time. Time best spend in search of the Lich’s phylactery. In time the Lich will reform and he will remember. He will not be so easy to kill a second time. And this can and will go on and on until you find his precious phylactery and kill him for all time. The Phylactery is the magical receptacle that houses the soul of the caster and until it is destroyed the Lich will live on.
This level of power draws more than just humanoids to seek Lichdom. Often Powerful beings such as dragons are drawn to the magic in an effort to extend even their tremendous years beyond that of nature.  Unlike most other forms of undead creatures, the Lich retains all of the memories, personality, and abilities that it possessed in life — but it has a virtual eternity to hone its skills and inevitably becomes very powerful.

Over the years and editions how has the Lich looked in D&D.. well Simply scary as hell ! Back in the day in AD&D.  He had a paralysis touch. And that was if he let you off easy.  If you were 5HD or lower you just ran .. Bye see ya.  And he had spells all the way out to 9th level.  Who wants to face off with an NPC that can open up with Mordenkainen’s Disjunction and then start chucking disintegrates and cloud Kill.. And if he needs to he can fly around overhead and throw fireballs and the list goes on… Have a nice day and have fun re-rolling your Party.
In 3.5 they make Lich a Template that you can just drop over any humanoid that can create a phylactery.. Shudder. It just makes what ever you drop the Template on.. Way more powerful.
In 4E Not only was a Lich pretty awesome still.. but you could become one as a PC if you met the requirements to cast the ritual…                                (DM Note: NO BAD PLAYER!)

As far as D&D5E goes. I know I may have some players reading this right now.. If you are. And If I ever, EVER throw a Lich at you.. RUN ! The Lich in 5E is of the power level that his lair has actions in combat that the Lich can invoke every round of combat. In addition to his own legendary actions. He is no longer the soft template That he has been for the last few generations. Now he is back to form and will deal death like a machine gun to his opposition. Not only does he have an amazing array of spells but his lair can allow him to roll a D8 every round to regain a spell level cast of the rolled level.. he is eternal, Evil and he will lay the smack down on you if you are unprepared.
And he still has a phylactery… Happy Gaming !