Demilichdom
A lich's phylactery is a fail-safe. When slain, a lich's soul travels to the phylactery instead of the afterlife, and it can then be reformed in a new body. If a lich's phylactery is destroyed or otherwise lost, the lich's hold over its soul is weakened, and it is unable to prevent its body from fully decaying and crumbling to dust over the years. Only the skull remains, containing the necromantic power that still suffused its body. The lich's items are absorbed into its new form, and the concentrated magic solidifies into gemstones that replace its teeth or eyes, or jut out of the top of its skull. However, it loses the ability to cast spells.
The loss of the lich's phylactery causes it to slip into a slumber, reawakening only when living creatures draw near and it feels a desire to consume life. The demilich's raw power may be greater than when it was a lich, but in exchange it loses it ability to actively work to accomplish its goals, becoming a husk of who it once was, almost constantly inactive. Fortunately for the lich, this leaves its mind free to wander the universe, learning new, more dangerous secrets. If the demilich is destroyed, it will no longer reform, but even then it may not be gone forever. Its mind, now free from all ties to the material world, may eventually regain a physical form, or find new ways to enact its will even without one.
In general, though, a demilich is a reminder of the dangers of tampering with the natural order of the universe. Doing so may grant on a temporary escape from death, but at the cost of a tortured existence devoid of what made life so precious in the first place.
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