Hollowing in Dark Souls 3 is accumulated when the player dies while branded with a Dark Sigil. The amount of Hollowing gained per death is equal to the number of Dark Sigils in the player's inventory. At 15 or more Hollowing, the player becomes Hollow.
Speaking with Yoel of Londor allows the player to gain a free level five times in exchange for being branded with a Dark Sigil each time. While the first level up needs no hollowing, the following need respectively: 2, 6, 12 and 15. The player becomes a hollow when the hollowing counter reaches 15. Three additional Dark Sigils can be obtained from Anri of Astora through Yuria of Londor's questline, but these do not give the player any more levels.
All Sigils appear under Key Items, and each hollows the player for one point in death. For example, when three Dark Sigils are in the player's inventory, the player will gain 3 more Hollowing each time they die.
Once Yoel has given all five Sigils, he dies and Yuria of Londor appears where he used to be.
Removing Dark Sigils and Hollowing
Once the player has given the Fire Keeper the Fire Keeper Soul, she can remove the Dark Sigil from the player and remove all Hollowing, which costs a number of souls equal to the player's last five levels. The levels are not lost, the player is just paying the cost for the previously "free" levels. This will cancel Yuria's questline, Londor Pale Shade's summon signs, and make the Londor Pale Shade invade the player at certain in-game areas. Note that curing the Dark Sigil in this way will make it impossible to get any more Dark Sigils for that playthrough, and the player will not be able to get the Usurpation of Fire ending.
The player can reset their Hollowing counter back to zero by using a Purging Stone. Purging Stones will only reset the player's Hollowing and do not remove the Dark Sigil, and using a Purging Stone will not abort Yuria of Londor's questline. Purging Stones can be purchased from Yuria of Londor, or bought from the Shrine Handmaiden if you give her the Hollow's Ashes or Yuria's Ashes. They can be farmed from Sewer Centipedes in the Profaned Capital toxic swamp.
Alternatively, it can be healed by Velka if you select the Dissolution option.
Obtain info about the Hollow infusion on the Upgrades page.
Your character will lose his/her shadow when hollowed (patch 1.03)