Helpless

A helpless character or creature is held, bound, sleeping, has any of its abilities reduced to zero, or otherwise completely at an opponent's mercy.

Being helpless
A helpless character or creature cannot attack, cast spells or SLAs, drink potions, activate items or class abilities. Depending on the source of helplessness, you might be able to move. You can (and should) block to reduce incoming damage.

A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target get a +4 bonus (equivalent to attacking a prone target). Ranged attacks gets the same bonus. Rogues and characters with Backstabbing items can always sneak attack helpless targets if they are subject to sneak attacks regardless of aggro.

Extra damage
Not all sources of helplessness are equal. Some particularly nasty effects can cause the target to take extra damage.

Players that have been rendered helpless take additional damage. The value varies depending on dungeon difficulty:
 * Solo/Casual: +5% damage
 * Normal: +10% damage
 * Hard: +15% damage
 * Elite: +20% damage
 * Epic: +25% damage

Helpless monsters take an extra 50% damage from all sources - including sneak attack damage, spells, etc. (except glancing blows). Few rare abilities exist that increase your helpless damage:
 * Brutality: +20% when using a two-handed weapon
 * Bully: up to +15%
 * No Mercy: up to +30%
 * Sense Weakness: up to +30%

Overcoming helplessness
Few rare effects exist that can help you overcome some of the sources of helplessness.
 * Unstoppable
 * Forced Escape
 * Unstoppable
 * Forced Escape

Miscellaneous information
Note that paralysis from a paralyzing weapon will prevent action but not cause this condition, contrary to one of the loading screens.

Helplessness is often caused by a spell such as Hold Person or Sleep. In the case of a Hold Person spell, casting Freedom of Movement will NOT end this condition, but will prevent future Hold Person spells while it lasts. To remove hold spells, use the spell Remove Paralysis. In the case of Sleep - you'll only get a single hit with the +50% damage, as attacking a sleeping target will wake it up.

If helplessness is caused by ability score attribute damage, it can be cured by any Restoration spell or Heal. Attribute damage also recovers with time.

History
Before Update 9, helpless opponents did not take extra damage. Instead, every hit was a crit.

Coup De Grace
Coup De Grace is a related Pen and Paper DnD feature that does not directly exists in DDO. Instead for things which would qualify for a Coup De Grace in Pen and Paper, qualify for helpless +50% damage bonus in DDO. Devpost on reasoning for this:

Note: Auto-crits were later replaced with the +50% damage bonus.