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In Commander, with 99 card decks, having access to searching out whatever card you need for a situation is powerful, and even though Solve The Equation is a weaker Mystical Tutor, it's still a phenomenal card worth running
You must discard from play the relevant which are implied from the name, and summon this card from an extra deck you have, in other words, not the deck you're playing with
Umbral Horror Will o' the Wisp X DARK Level 1 [ Fiend / Effect ] ATK 0 DEF 0 When this card is Normal or Special Summoned: You can target 1 "Umbral Horror" monster you control or in your Graveyard, except this card