Tar Skulls remain in place, never moving from the spot from which they emerge from the tar and snapping their heads forward to attack. They are often out of range of the player's weapons, striking one requires alchemy or baiting them to attack, bringing their heads into range. Their attacks are more powerful than those of the Maggots and Skelesnails, making them quite dangerous.
This monster has the -30 Hit % attribute. For more info about that attribute, go here.
Tar skulls are not particularly useful for player improvement. They have more hit points than either Skelesnails or maggots, and drop fewer items. This, combined with the difficulty in hitting them, means it will take a comparatively longer amount of time to defeat them, mitigating the benefit of slightly higher experience and Talons they offer.