In terms of gameplay, Hollow refers to an in-game mechanic which limits the abilities and possible scenarios of the Chosen Undead. Namely, not being able to summon NPCs and other players in the form of white phantoms. Alternatively, being Hollow might also be desirable at times as it prevents the player from being invaded by other characters and players.
It is possible to "Reverse Hollowing" by offering Humanity at a bonfire; however, death results in reverting to a semi-Hollowed state. Also, the darksign can recreate the Chosen Undead's body in order to be reborn.
Hollowing in Dark Souls II is similar to the previous game, with a few key differences:
- Being hollow does not prevent being invaded by NPCs and other players.
- Each death suffered will reduce the player's max HP in increments of 5%. This will continue until it caps at 50% of maximum HP, unless the player has a sin level of wretch, which will cap it at 95%. With a Ring of Binding equipped, the HP reduction caps at 75% of the player's health.
- The player's appearance visually degrades with each death.
Reverse Hollowing can be made using a Human Effigy from the inventory, not burning it at a bonfire.
Hollowing is a bit different in Dark Souls III compared to the previous games.
- Hollowing does not occur until the Ashen One has acquired at least one Dark Sigil. Every time the player dies, their hollowing stat will increase by an amount equal to the Dark Sigils in their inventory. The Ashen One's body will start decaying after reaching a certain level of hollowing, but this effect is purely aesthetic.
- Hollowing has no negative effects on gameplay. However, it is required for certain NPC questlines and provides a bonus to luck when wielding Hollow weapons with a hollowing level of at least 15.
- Hollowing can be temporarily cured by paying the Statue of Velka to reverse hollowing, or by consuming a Purging Stone. To remove the Dark Sigil, and hollowing, permanently, the player must give the Fire Keeper Soul to the Fire Keeper, and then pay her an amount of souls relative to the player's level of hollowing. Healing the Dark Sigil cannot be undone, and will stop questlines that require it.
The symptomatology of being Hollow is primarily characterized by a lack of free will. Those that are Hollow may still have attractions to things they valued as living and Undead. This might explain why so many have maintained stations at various posts as well as their aggression towards non-Hollows.
Beyond this, drastic changes in physical appearance can be observed. Skin appears slightly decomposed and the body thoroughly dehydrated, prominent veins can be seen surrounding the heart although physical signs may only be indicative of progression into a Hollowed state and not of being entirely Hollowed.
The exact process of turning Hollow is not entirely clear. Many Undead characters encountered in Lordran will eventually turn Hollow if certain prerequisites are met. Most characters are fearful of going Hollow and try to ward it off as much as possible, though the Crestfallen Warrior seems to find the prospect of going Hollow to be desirable.
The Chosen Undead is already in a somewhat Hollowed state when escaping from the Undead Asylum and seems to be subject to a different progression pattern compared to other Undead characters in Lordran. Despite the Chosen Undead's appearance as a Hollow, he or she is able to maintain their free will and this is immediately recognized by other characters.
In general, it seems that Undead go Hollow more quickly when they have no further goals or purpose. Even characters like Blacksmith Lenigrast who are visibly almost fully Hollow can maintain their lucidity if they are focused on their craft. This is likely why the Chosen Undead never fully hollows during the course of the game, since he/she will continue on their journey for as long as the player is motivated.
At some point in time, Hollows develop their own civilization, most notably Londor, governed by the Sable Church.