The Crypt Thing is a classic monster, dating back to at least the second edition of Dungeons and Dragons. Though its appearance and name were most likely inspired by the old TV show Tales of the Crypt, it had a set of abilities that made it a much more interesting opponent than its comical source material would imply.
Another thing that made the Crypt Thing stand out for me was that it was a non-evil undead that could be talked to and reasoned with. While such things of course vary from setting to setting, I generally prefer this take on the undead to the "Always Chaotic Evil" description that most of them are printed with. I believe I'll be making a longer post about the different possible ways to portray undead in your setting (and the various world mythologies you can draw from in doing so) someday, but for now, here's the 4E Crypt Thing.
Level 10 Controller (XP: 500)
Medium Undead Humanoid
Initiative: +7 Perception: +15 (darkvision)
Hit Points: 82; bloodied 41
AC: 24 Fortitude: 21 Reflex: 21 Will: 23
Resist: 10 necrotic
Vulnerable: 5 radiant
Melee basic Touch of the Crypt (at will); +14 vs. Fortitude; 2d8+5 necrotic damage and ongoing 10 (save ends).
Close burst Banish (at will); targets one creature within 6; +14 vs. Will; 2d6+5 psychic damage and teleport target to another square within range.
Close burst Offensive Transposition (recharge 4, 5, 6); targets two creatures within 6; +14 vs. Will; 2d8+5 psychic damage; if both targets are hit, they switch places and grant combat advantage until the beginning of the Crypt Thing's next turn.
Strength: 14 Constitution: 16 Dexterity: 14 Intelligence: 14 Wisdom: 20 Charisma: 18
Alignment: Unaligned Languages: Common
Crypt Things are patient, undead watchmen who have vowed to guard a certain location from intruders. When their wards are not being invaded, Crypt Things become inert corpses, their souls going back to wherever it is that the dead go. The instant the Crypt Thing's perimeter is breached, the spells that created the Thing will recall its spirit back to the mortal world, where it animates its old skeleton to deal with the interlopers.
Despite their vigilance, Crypt Things retain all the knowledge and intelligence of their previous lives, which means that many can be reasoned with. Some Crypt Things have vowed to only let certain beings enter their wards, or only beings on a specific mission; convincing the Crypt Thing (truthfully or otherwise) that you are one of the above might allow you to avoid a deadly battle.
In appearance, a Crypt Thing is most likely to look like a well-preserved skeleton, often wearing expensive ceremonial garments or ornaments.
Crypt Thing Lore (Arcana, Religion):
DC 20: Some ancient civilizations used ensorcelled, living corpses to guard tombs and treasuries from would-be looters. One of the most advanced types of these undead, the Crypt Thing, is fully sapient, and has the power to teleport intruders away from its ward.
DC 25: Crypt Things are created from the bodies of those who willingly vowed to guard the structure after death. In their undead state, these guardians retain their mortal identities and views, returning from the afterlife only briefly when their ward is being violated. While active, a Crypt Thing has control over the physical boundaries of its ward, which enables it to remove objects and creatures that don't belong in it. It also has a deadly touch that causes flesh to age and necrotize.
Once a Crypt Thing has decided that you don't belong in its tomb, it fights until destroyed. The Crypt Thing will use Offensive Transposition as often as it can, usually switching the most heavily armed and armored enemy with the one that appears physically weakest. When it cannot use this attack, the Crypt Thing will try to get close enough to an enemy to use Touch of the Crypt, shifting away as soon as it can.
If the Crypt Thing is getting overwhelmed in melee, it will fall back on Banish to cast the character who has done the most damage to it away. It will also use Banish if it is denied the opportunity to use either of its other attacks, usually teleporting its target into a square that is flanked by the Crypt Thing's allies.
A Crypt Thing is most likely to be found alongside other undead guardians, such as zombies or mummies. In particular, a Crypt Thing may be guarding a tomb complex inhabited by the mummified bodies of the royals it served in life. Note that Crypt Things do best when pared with Soldier and Brute class monsters, so that they can teleport enemies into their midst. They can also be hilariously amusing when paired with Artillery, using Banish to keep melee characters away from the more fragile targets.