Spine of Eldersblood can be purchased with the Alduin's Apocalypse deck or purchased alone for 100 soul gems. This card may also be acquired through random Heroes of Skyrim packs that are rewarded through game rewards or in-game purchases.
Spine of Eldersblood should be summoned as soon as possible to make use of its extra Magicka quickly. Otherwise, it is a very powerful creature. It can be used to deal massive amounts of player damage or kill strong enemy creatures without it dying.