Forgive me if this has been asked before. While replaying Dawnguard I was in the Soul Cairn and noticed huge crystals, on top of some structures. Everytime I'm near them they drain my health. I remember Valerica or Serana saying some people said that the Ideal Masters look like crystals. My question is: Are the huge crystals that drain my health the Ideal Masters?

They may be. In Battlespire the Ideal Masters were portrayed as crystals, but Dawnguard suggests that the crystals may not be the Ideal Masters themselves, but rather conduits through which they work. The actual truth isn't known.