There are several different levels of undead that creatures can be.
Partial Undead
You count as being undead for the purposes of abilities, damage, and other effects. You also gain Healing Sensitivity.
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Minor Undead
You count as being undead for the purposes of abilities, damage, and other effects. You also gain Ethereal Senses, Healing Sensitivity, Slow Mana Drain, Undead Mind, and Undying Durability.
Full Undead
You count as being undead for the purposes of abilities, damage, and other effects. You also gain Ethereal Senses, Healing Sensitivity, Impure Durability, Mana Drain Grasp, Undead Mind, and Undying Durability.
Greater Undead
You count as being undead for the purposes of abilities, damage, and other effects. You also gain Ethereal Senses, Healing Sensitivity, Magical Durability, Mana Drain Touch, Undead Mind, and Undying Durability.