There are several different levels of undead that creatures can be.
You count as being undead for the purposes of abilities, damage, and other effects. You also gain Healing Sensitivity.
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.
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.
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.