Abyssus in Path Of Exile is a unique ezomyte burgonet that when equipped grants the player up to 25 additional points towards all three attributes, up to 225 percent additional melee critical strike multiplier, up to 120 percent increased armor, and adds up to an additional 60 physical damage to attacks. The downside to using this unique is that it increases the amount of physical damage you take by a minimum of 40 percent. This unique has a maximum armor rating of 722 if all armorer's scraps have been applied to the unique. This unique has changed since it's introduction into the game in April of 2013.
Item Usefulness Information - This unique is best suited for melee based character builds and range based character builds that are focused on dealing as much physical and critical strike damage as possible. This unique is not for character builds that are focused on having high amounts of physical damage based mitigation and physical damage based survivability. The up to 25 additional attribute points granted by the unique is beneficial towards all character classes, since it can help ease
attribute requirements on gear and grant small increases towards maximum life, melee physical damage, accuracy rating, evasion rating, maximum mana, and maximum energy shield. The up to 225 percent additional melee critical strike multiplier granted by the unique is beneficial towards melee based critical strike builds. To gain the full benefit of using an Abyssus as a melee based character, you will have to not spec into Resolute Technique, as the keystone passive skill will make your melee based character never deal critical strikes, which completely removes the benefit of the unique modifier. The up to 120 percent increased armor is great for any character builds that scale armor as their main source of defense against enemies. The up to an additional 60 physical damage added to attacks granted by the unique is enormous and best in slot for character builds that want to deal as much physical damage as possible. The minimum 40 percent increased physical damage taken granted by the unique is it's one downside, however this downside can be mitigated by having a large amount of life, a large amount of armor, and utilizing the Vaal Pact keystone passive skill in tandem with a large amount of leech. You cannot use this unique in the early stages of the game, since it's level requirement is level 60. In the late stages of the game, this unique is great for character builds that are focused on dealing physical and critical strike damage, and due to it's unique modifiers, is likely the best in slot helmet for maximizing damage per second.
Informative Links -
http://pathofexile.gamepedia.com/Abyssus - additional information regarding the Abyssus unique
http://pathofexile.gamepedia.com/Resolute_Technique - additional information regarding the Resolute Technique keystone passive skill
http://pathofexile.gamepedia.com/Vaal_Pact - additional information regarding the Vaal Pact keystone passive skill