Darkside materia is a 
Command Materia in Final Fantasy VII Rebirth. Its ability, Darkside, costs one ATB charge and grants the darkside effect to the user (or removes it if it is already active). This increases the potency of their abilities, both weapon abilities and those from equipment, but drains HP on each use.
Darkside materia is introduced in Rebirth, and did not appear in Final Fantasy VII or Final Fantasy VII Remake. It is based on the recurring ability seen throughout the Final Fantasy series, typically used by Dark Knights, that drains HP to deal damage.
Obtained[]
The darkness materia can be bought Chadley's Materia Development with 40 Nibel Region data points, available from chapter 11, "The Long Shadow of Shinra".
Stats[]
| Level | AP required | Effect |
|---|---|---|
| 1 | 0 | Increase ability potency by 7%, suffer 5% HP damage each use |
| 2 | 300 | Increase ability potency by 10%, suffer 5% HP damage each use |
| 3 | 600 | Increase ability potency by 15%, suffer 5% HP damage each use |
| 4 | 600 | Increase ability potency by 17%, suffer 5% HP damage each use |
| 5 | 900 | Increase ability potency by 20%, suffer 5% HP damage each use |
Use[]
Darkside increases the potency of the user's abilities, but causes them to drain 5% HP each use. The Darkside icon appears next to affected abilities before they are used. The ability is a risky but effective way to increase the user's physical damage output (as while it can increase magic damage output, most magic damage comes from spells, which are unaffected by Darkside).
Darkside is best used as a means of boosting physical damage output beyond normal means. Especially when combined with 
Genji Gloves, it allows many basic attacks to become extremely potent. Darkside can be further complemented by 
Berserk, which will also increase physical damage output as well as damage taken, but also by means of boosting 
Attack Power and 
Strength. 
Elemental Materia linked in a character's weapon also boosts damage even if the enemy is neutral to the granted element. Darkside can be especially good used before an enemy staggers.
Darkside is best used with party members with high strength but also high durability. Red XIII is a durable member who can heal himself with his attacks such as Siphon Fang, and thus can make use of Darkside more safely, while Cloud and Barret cannot heal themselves with normal attacks but have great durability to make Darkside more useful on them (and in Cloud's case, a high strength). Tifa is less durable, but she has the highest strength, and her ability Divekick can be pushed to deal immense damage with Darkside, especially with 
Bravery active, making her a viable candidate if the risk is mitigated through other means.
A strong move for Cloud is mid-air Braver, boosted by bravery, Prime Mode berserk, and Darkside. A character equipped with 
Auto-Weapon Ability Materia + HP Absorption Materia, combined with Darkside Materia boosts the potency of the abilities while negating the HP loss.
The player can bestow the user with 
Regen to help them get some HP back when using Darkside.