Black skulls will appear when a player kills double the amount of players needed to attain a red skull. This is used as a replacement of the old 30-day ban for "Excessive unjustified player killing.
A player gets a black skull if they kill:
- Six or more unmarked characters in one day (24 hours),
- Ten or more unmarked characters in one week (7 days), or
- Twenty or more unmarked characters in one month (30 days)
When a player has a black skull they will no longer be able to attack an unmarked player - that is any player without a skull. In addition to this, a black-skulled player may not use area Attack Spells and Attack Runes (i.e. Hell's Core, Explosion, etc.), as well as Burst or Diamond Arrows to prevent black skulls from killing unmarked players indirectly. They also cannot summon or convince creatures, or use Undead Legion or strike spells (i.e. Energy Strike) which are not aimed at a target.
A black skull will remain on a character for a total of 45 days. Whenever a black-skulled character is killed, he/she will respawn in temple with 40 hitpoints and 0 mana. Black-skulled character, like a red-skulled, who dies will drop all items regardless of whether they have blessings or wear an Amulet of Loss. Black-skulled character will receive 100% damage in PvP combat instead of 50%.
Source: https://tibia.fandom.com/wiki/Skull_System#Black_Skull