When timed correctly, this attack will connect to the opponent and start draining their health while restoring the player's own health. It only works on NPCs and players. Other enemies will simply take a burst of damage if hit with Lifedrain, nullifying the grab, health restoration and a considerable chunk of the damage. Due to the Dark Hand's fast swings, it should for this reason only be used against enemies that can be grabbed.
Lifedrain's damage and heal are both based on the user's intelligence and faith, and is not affected by the Dark Clutch Ring, Hornet Ring, or the Pontiff's Right or Left Eye. However, it does activate Aldrich's Rings. This means that combining Aldrich's Sapphire with the Farron Ring will make Lifedrain restore two FP per use, assuming the attack connects properly.
If timed properly, Lifedrain can deal significant damage while also healing the player for an equally significant amount. Additionally, it will leave the opponent wide open for consecutive attacks. Furthermore, the player and the victim are both invulnerable for the duration of the animation, meaning Lifedrain may be useful if fighting multiple enemies at once, especially if they're all human, as the chances of an attack connecting become much larger.
However, missing the skill will leave the player equally defenseless as the animation itself and recovery are long, and the skill seems to not provide any active poise frames. The latter weakness can be solved by using any source of passive poise, such as Iron Flesh, Perseverance or Stone Flesh.
Due to being slow and well-telegraphed, Lifedrain is difficult to hit in PvP. However, should it connect, it will cause heavy damage to the opponent. It can also be useful as a way to conserve Estus Flasks.
Additionally, due to being a grab, Lifedrain ignores poise. It can also be combined with Iron Flesh, Perseverance or Stone Flesh as all of them provide passive poise and reduced damage for their duration, allowing the wielder to use Lifedrain for trades, likely greatly increasing the likelihood of the attack connecting while also restoring a large portion of the HP they lost in the trade.