Ghost Damage

Ghost damage is a (Bug/glitch/intentional) mechanic sprouting from ferocity. Ghost Damage occurs when Ferocity procs on bosses with a leaderboard .(such as Endstone Protector, Arachne, Dragon, Magma Cube Boss, Headless horseman [not to be confused with Slayer Bosses] ) All damage done as Ferocity will not be counted towards defeating the boss, but will be counted as damage on the leaderboard.

Note
Although this can be used to get a good placement on a boss, because it doesn't contribute to defeating the boss, it can make Bosses such as Arachne take awhile if all players are focusing on ferocity and not maximizing Damage itself.

Ghost Damage only affects the resulting damage off of ferocity, not the original hit. (Say you do 1000dmg to a boss, and ferocity procs once, 2000dmg will be counted on the leaderboard, but the bosses health will only go down by 1000)