Blood Fang is Red XIII's first level 2 Limit in Final Fantasy VII. It deals damage equal to 1.25 times Red XIII's normal physical damage, but heals his HP and MP for 25% of the damage dealt. Using it seven times unlocks Stardust Ray.
When the attack is performed, Red XIII engulfs himself in energy and then dashes through an enemy.
Blood Fang is obtained by killing 96 enemies with Red XIII. To speed up the process, he can equip the Enemy Skill Materia, particularly with the Matra Magic ability, to kill enemies in the Grasslands Area, in the Mythril Mine, or in the forests of the Junon Area (linking Magic Materia to All also works). Later on, the large forest in the Nibel Area past Mt. Nibel is a useful location, with 4.4 enemies per encounter, including a wave of six enemies (Battery Cap).
Blood Fang deals low damage, but is much more useful for its healing effect. The HP and MP absorption provide great sustainability for Red XIII, and the MP gain after the attack is particularly useful as he is best used as a spellcaster due to his stats; recovering MP will allow him to cast much more often as it can potentially refill his MP.
Overall, the low damage dealt by Blood Fang means that it falls off later on in comparison to his other Limits. His second level 2 Limit, Stardust Ray, deals 0.625 times Red XIII's normal damage spread over 10 hits, totaling over 6 times his normal damage spread across all enemies hit. Once obtained, this becomes a much more useful ability unless Red XIII sorely needs healing or recovering MP.