Final answer:
Sectumsempra is a slashing curse meant for enemies, described as Option 1 in the Half-Blood Prince's potion book, and causes deep wounds.
Step-by-step explanation:
The description of Sectumsempra in the Half-Blood Prince's potion book is Option 1: For enemies - a slashing curse. This curse is designed to slash the target as if it had been struck by an invisible sword, causing deep wounds.
It is not intended for healing, creating illusions, or summoning fire. The curse was used by Harry Potter in the Half-Blood Prince.
If he found the spell in the margins of the Potions book previously owned by the mysterious Half-Blood Prince and used it against Draco Malfoy without fully knowing its effects.