Incorporeal

Incorporeal creatures cannot be easily hit with normal weapons, which suffer a 50% chance of automatically missing the target.

Implementation in DDO
There's no such thing as touch attacks in DDO, which is how incorporeal undead attack in D&D. To compensate for this, developers gives them very high to-hit bonuses to simulate it. They can miss, but it takes substantial Armor Class to get it to be reliable. 

Incorporeal monsters
Incorporeal creatures include: and
 * Ghosts
 * Quells
 * Reapers
 * Shadows
 * Spectres
 * Wraiths
 * some Wheeps - only those found in Accursed Ascension

There are also unique, named monsters who have the incorporeal trait (many of which are of the above races), and sub-types of usually corporeal monsters such as Doomspheres, Dream Scourges, Umbral Gargoyles and Umbral Worgs, some Liches, and so on.

These monsters are affected by Incorporeal bane weapons.

Note that, despite the name, the Ghostly Skeletons in Delera's Tomb are not incorporeal.

Player characters as incorporeal
Players can gain an Incorporeal miss chance via:

10%

 * Feywild Tap (Elf enhancement) - 10%
 * Ghostly (item enchantment) - 10%

25%

 * Dark Discorporation (Warlock spell)
 * Shadow Veil (Ninja Spy 3rd core enhancement)
 * Ghostly Essence (Shadar-kai enhancement)
 * Shadar-kai past life feat (temporary effect)
 * Shadow Form (Shadowdancer epic destiny)
 * Shroud of the Wraith (Pale Master 4th core enhancement)
 * Walk with the Spirits (Primal Avatar epic destiny)

35%

 * Shroud of the Wraith with Improved Shrouding (Pale Master T5 enhancement)

50%

 * Reborn in Light angelic form

+5% stacking

 * Whirlwind (Ranger enhancement)

Bypassing the incorporeal miss chance

 * Ghost Touch and Ghostbane weapons bypass the Incorporeal penalty.
 * Paladins can take the Slayer of Evil II enhancement, which provides Ghost Touch for all weapons.
 * Wizards can take the Shroud of the Wraith enhancement, which provides Ghost Touch for all weapons.
 * Aasimar Scourges gain Ghost Touch on all attacks; requires active Bond of the Scourge and a proper enhancement.
 * Vistani can gain an attack that grants Ghost Touch for 10 seconds.
 * Players can wear an Ethereal or Ghostly item to have any of their weapons defeat Incorporeal.
 * Ethereal items
 * Ghostly items
 * Piercing Clarity - tier 4 ability of Grandmaster of Flowers
 * Named augment:

Tip: Incorporeal Bane items increase the damage dealt to incorporeal enemies, regardless of their race. However, in itself, this enchantment does not allow a weapon to bypass incorporeal miss chance. Fortunately, on named weapons, this Bane effect is often combined by Ghost Touch enchantment.
 * Named Item: provides Greater Incorporeal Bane.
 * Named Item: provides Lesser Incorporeal Bane.
 * Named Item: provides Greater Incorporeal Bane.