Life Drain can be referring to either a special effect or a modifier.
Life Drain, Lifedrain or Life Steal is a special effect that allows characters to gain Health when they deal damage.
On the other hand, Gauntlets of Azrael and Overpowered 5-U-93-R Pill which has a high chance of healing any character and their teammates by 100% of the damage dealt by their Combo ender; additionally, LexCorp Chest Armour V2 has up to 99% chance to life drain upon using an SP2!
Martian Manhunter/Blackest Night's passive has a life drain phase that allows him to heal his whole team by full damage dealt by most sources for about 6 seconds.
Life drain is also in The Joker/Batman Ninja's passive, where knocking out an opponent allows him to deal life drain on all attacks while healing from enemy life drain for a limited time. While on the other hand, Batman/Batman Ninja can be immune to life drain as part of his passive.
Life Drain (called Vampire during the Red Lantern Hal Jordan and The Flash/Earth 2 challenge boss fights) is also one of the Modifiers where opponents gain health equal to a percentage of the damage they deal. The percentage varies depending on the match in question. Life Drain can be encountered in Challenge Mode, Survivor, or regular standard battles.