What is dark souls reverse hollowing?

In the game Dark Souls, hollowing is a curse that slowly drains away the character's humanity, turning them into a mindless zombie-like creature. To reverse hollowing, the character must consume a Humanity item or successfully defeat a boss as a phantom.

When the character is in a hollowed state, their maximum health is reduced, and they are unable to summon other players for co-op play or invade other players' worlds. However, when the character restores their humanity, they regain full health, become able to interact with other players, and can use certain powerful items that require humanity.

Reverse hollowing is essential for progressing through the game as it unlocks the ability to summon other players for assistance in boss fights and opens up new areas of the game. It also significantly improves the chances of survival in dangerous situations.