Valdemar The Cursed was once a powerful warchief and legendary king who led a fight against forces of Helheim. The warchief was supposedly died a glorious battle in said campaign, only to return as a Draugr.
Valdemar acts as a special boss that can be encountered in Conquest. Being a warchief in life, he is a powerful adversary as formidable as powerful enemies such as Wyvern. Too add the threat he pose, he holds a command over Dark Vaettir who accompany him and acts as bodyguards. As long as there's at least one Dark Vaettir around, he is invulnerable.
Defeating him is the objective of The Cursed battle.