You can Special Summon this card (from your hand or Graveyard) by removing 6 Spell Counters from a "Magical Citadel of Endymion" you control. If Summoned this way: Target 1 Spell Card in your Graveyard; add that target to your hand. Once per turn: You can discard 1 Spell Card, then target 1 card on the field; destroy that target.
Citadel can be used to special summon Master Magician from your hand or graveyard when you have 6 spell counters.
Could be used in a Spellbook deck with their high spell card count. Spellbooks are also generally good at supporting Spellcaster-type monsters. Spellbooks can easily search each other from the deck, therefore they can be used as cost for Endymion's destruction effect.