Lifesteal is a special type of Creature Spell used by some bosses. The Lifesteal is not only a direct nor an area attack, but rather depends on the positioning of the players near the boss. It seems to be used on a target, but it will "spread" to players that are near each other, resembling a chain effect. As the name suggests, the damage dealt by Life Steal is returned to the caster as health, so it can be seen as the creatures' version of the Vampirism (Life Leech) Imbuement, but specific to one attack.
Similar to Damage Reflection, the health regenerated by the creature will not necessarily be the same as the damage taken by the player, because Lifesteal has a fixed percentage of efficiency (for example, 150%). It's also important to note that only the damage caused directly to health is recovered as Lifesteal, so even though those using Magic Shield will also take damage, this damage does not heal the boss.
It seems to be possible to avoid the Lifesteal by standing far from other players in the same room, thus this should be tried when possible in order to minimize the damage taken and the boss healing.
The following messages appears on the server log when Lifesteal is used:
Creature healed itself for X hitpoints (lifesteal).
Player loses Y hitpoints due to an attack by creature.
List of Creatures that use Lifesteal
Note: Even though some bosses' summons also use this spell, only bosses are displayed here for simplicity.