Lesser Corruption is a spell within the Arceuus spellbook that requires level 64 Magic and the completion of A Kingdom Divided to be cast. It can only be cast on opposing players.
When cast, the player's next successful hit has a 33% chance of inflicting corruption onto the target. Targets under the effects of the Mark of Darkness spell will cause corruption to occur twice as often. If a player with corruption spells active attacks an opposing player with Ward of Arceuus active, the attacker's corruption spells will remain active until the target's Ward expires, after which the next successful attack post-expiration will roll the chance for corruption.
Players cannot corrupt a player if they are currently corrupted, nor can they do so while the target is under the effects of the Ward of Arceuus spell. However, there is no immunity period after corruption wears off on a target.