This card can be used to slow the rate at which a Lightsworn Deck is depleted.
Equip this card to Guardian Eatos for her effect and that card will add on top of your Deck.
Horn of the Unicorn is a stronger version of this card. However, it only returns to the top of the deck when sent from the field to the graveyard, while Sword of Deep-Seated will send itself to the top of the deck in other situations (e.g. discarded to activate Cursed Seal of the Forbidden Spell).
Since this card's effect that sends it to the Deck is obligatory, that shall slow down the Deck way too much. To amend that weak spot, use Freed the Matchless General's effect to forfeit your draw and grab a Warrior from your Deck instead.
Another way to help prevent deck slowdown is to equip it to Airknight Parshath. Since you draw a card every time it inflict Life Point damage, this combo results in a 2400 ATK monster with Piercing that allows you to draw the card that could be blocked by the Equip spell's destruction ahead of time.
Endymion, the Master Magician - Once per turn, you can discard 1 Spell Card to destroy 1 card on the field. This card could assure you the effect once per turn.
Use this card to counteract Vampire Lord's effect if your opponent chooses Spell Cards. This card will go into the Graveyard, then return to the Deck, which will protect your Spell Cards as your opponent won't want to bother choosing that type of card for the effect after the first attempt.