Incorporeal

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

Concealment and Dodge are different mechanics that provide a separate miss chance. See Miss chance.

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:
 * Ghosts
 * Quells
 * Reapers
 * Shadows
 * Spectres
 * Wraiths
 * some Wheeps - only those found in Accursed Ascension
 * some Monster Champions

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 from various sources.

However, note that Incorporeality and Concealment have decreased effect in Reaper difficulty. This reduction begins with 10% Concealment and 5% Incorporeal bypass at Reaper 1, with a further 5% Concealment and Incorporeal bypass per Skull difficulty.

5%

 * Ascendant Shroud (Pale Master enhancement) - 5%

10%

 * Fey Energy Tap (Elf enhancement)
 * Ghostly (item enchantment)
 * Shroud of the Wraith (Pale Master enhancement)

15%

 * Enhanced Ghostly (item enchantment)
 * Misty Escape (SLA, Warlock level 15 with Fey pact)

25%

 * Dark Discorporation (Warlock spell)
 * Shadow Veil (Ninja Spy 3rd core enhancement)
 * Ghostly Essence (Shadar-kai enhancement)
 * Shadar-kai past life feat (temporary effect)
 * Depths of Darkness (Shadowdancer mantle upgrade)
 * The Shroud of the Wraith can be upgraded up to 25% incorporeal through enhancements in the Pale Master tree.

60%

 * Greater Form (Sky or Thorn) (Primal Avatar epic moment)

+5% stacking

 * Whirlwind (Ranger enhancement)
 * Subtle Force II (Eldritch Knight enhancement)

Bypassing the incorporeal miss chance
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. On some named weapons, this Bane effect is combined with Ghost Touch enchantment.
 * 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 get Undead Augmentation, which includes Unhallowed Touch, providing Ghost Touch for all weapons.
 * Rangers and Elves who take the Arcane Archer Force Imbue enhancement gain force damage AND makes your arrows Ghost Touch.
 * Wizards and Sorcerers who take Eldritch Knight Spellsword gain Ghost Touch on their weapons (even ranged ones!)
 * 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:
 * Divine Sphere Epic Destiny feat Holy Strike
 * Named Item: provides Greater Incorporeal Bane.
 * Named Item: provides Lesser Incorporeal Bane.
 * Named Item: provides Greater Incorporeal Bane.