Currently my main wizard is a level 55 in Celestia. I am soon getting Skeletal Dragon at level 58. The Wiki says you need to go into Nidavelliar to get the spell and unlock all other future death spells. Is this true? Is Nidavelliar in Griz or Wintertusk, and how far in is it? To be honest, I just want to focus on main questing and maybe eventually complete Griz and Winteusk. But can you do this while still obtaining Skeletal Dragon?
Before I bought Grizzlheim and Wintertusk I was fine without it. In the past year, I finally bought it and I am glad that I did. Some of the schools are very important spells that can be learned in Wintertusk such as Storm's spells. Their healing spell is vital later on. If you don't want to buy the two, you can always just buy Nidavellir or get a membership for it. I would highly recommend Wintertusk and Grizzlheim though.
Anyone can access Nidavelliar, even if they haven't quested there. On my first wizard (Death) I didn't do Grizzlehiem/Wintertusk until I was level 90. However on my fire wizard I did Grizzlehiem/Wintertusk during the middle of Celestia, and I highly recommend you do the same. It gives you enough experience to level up a few levels to give you a better advantage in Celestia.