Celebi decks can now have Serperior out way earlier, while Mewtwo decks can more consistently land Gardevoir
Dark Sanctuary also applies a Fairy Box-like effect with a 50/50 chance of negating each opposing monster's attack and inflicting burn damage equal to half that monster's ATK
Once per turn, if a Pendulum Monster Card(s) in your Monster Zone or Pendulum Zone would be destroyed by battle or card effect, you can send 1 Spellcaster-Type monster from your Deck to the Graveyard instead
Is a separate letter
As for the X spells themselves, that includes a rich variety of Hydras such as Hydroid Krasis, which is overwhelmingly strong even without Zaxara there to generate additional value on the side, along with other staple Hydras like Lifeblood Hydra, Protean Hydra, and the mighty Genesis Hydra
Id definitely pick Obscura Operation over the rest if value is what youre going for