When near players it will "attack" the area around them occasionally with a non-damaging boulder which will change the appearance of all players in its radius into a Vampire, Ghost, Witch, Skeleton, Mummy, Pumpkin head, Phantasm, Pirate Skeleton, Dark Torturer, Blightwalker and Spectre. All changes per boulder are the same so all within the radius of one boulder will receive the same costume. This attack also can affect NPCs and creatures, including itself. You can not use it to train because it won't cause effect in your skills.
It's possible that this year the list has been incresed.
Source: Tibia.Fandom.com; Portaltibia.com.br; Tibiawiki.com.br