Aca esta como debe funcionar lichborne: Lichborne - Wowpedia - Your wiki guide to the World of Warcraft, y te lo copio y pego abajo.

Becoming undead has a number of consequences for the death knight:
Allows a death knight to self-heal using [Death Coil]
Makes the death knight susceptible to abilities able to target Undead mobs, such as [Shackle Undead] and [Turn Evil]
Prevents certain abilities from being used on the death knight, such as [Sap], [Polymorph] and [Hex]

Previene sap, poly y hex. Por ende si usas lichborne despues de que te usen algunas de esas 3 skills, no te lo va a sacar. Solo lo previene durante su duracion.
Y si usas lichborne con un fear o con el seduce de la succubo del lock, te lo remueve y te hace inmune a ellos por lo que dure lichborne.