Be careful not to attach meaning of lore Learned about the positive energy plane of ysgard with somehow Anybody telling you your character was illegal, because that was -not- the conversation.
she's no more illegal then a necromancer or shaggy's realm or a mage being able to summon negative energy spells. Technically, she's even more kosher on Ysgard because her Diety has direct control over death and underworld domains. So that domain and it's power is providing the source of negative energy to your cleric as it should be from her goddess. The key point Takeaway is: Your "source" is not from Ysgard!
Her whole calling is to corrupt Ysgard, so the fact that she is providing energy to source undead from her home plane is exactly perfect. You're role playing that cause for that goddess so you are not only legal on the plane, you are doing what we have actually asked clerics to do which is be the "forefront gospel message" for their Diety!
Now… If you were playing a Cleric of, say, Frigga and had taken The death domain… Then we would have a problem.
// not a team member but I did stay at a Holiday Inn express last night ....