Checking for Duplication

Each potential spell should be checked against the spells in the
Player's Handbook to see if it already exists. Considering that the PH lists hundreds of spells, re-inventing an existing spell is easy. A DM need not be obsessive about this; a large number of existing spells inflict damage or add bonuses and penalties to saving throws. As long as the new spell has a unique twist of its own, it can be deemed acceptable.

Of our examples, we notice that blood to dust is superficially similar to the 7th-level finger of death spell, since they both instantly take a victim's life. But the methods of achieving this effect are distinctly different; blood to dust changes the victim's blood to an inert substance, while finger of death snuffs out the victim's life force. Further, blood to dust is only effective on victims with blood in their veins. Blood to dust is different enough to stand on its own.

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