I would justify it by their connection with oshugun where there's a naruu inside and their connection to their ancestors who were righteous-like in life.
Or Maybe they would be some new form of shaman who decided to wear plate and use handed weapons and shields, but also use the elements of fire and life to create a psuedo light.
Or maybe they would learn from the argent crusade, which would be boring.
Orc paladins make alot more sense than undead paladins....lol. forsaken by definition makes no sense that they should be able able to wield the light, They're undead,
Theyre unholy creations weaved together by necromancy and unholy magic, something that is weak against the light, the undead would more likely burn to a crisp trying to wield the light.