Shrunken Heads can have 1-3 bonuses of +1-3 to Necromancer Skill Levels and can only be equipped by Necromancers. You normally hold Shrunken Heads like shields with a Wand, Dagger or another weapon. You cannot hold two Shrunken Heads at the same time on the same weapon tab.
Auto Mods are mods that "automatically" show up on all versions of an item, whether it's magical, normal, damaged, or rare. One Auto Mod from a given group is chosen for the item based on a random number and the level of the item (and the requirements of the Auto Mod). Many Class-specific items contain Auto Mods while others contain spell/skill bonuses.
Necromancer Shrunken Head Auto Mods
Blight - 2-7 Poison Damage Over 3 Seconds (1)
Venom - 5-14 Poison Damage Over 3 Seconds (15)
Pestilence - 13-18 Poison Damage Over 4 Seconds (25)
Anthrax - 20-50 Poison Damage Over 4 Seconds (33)
The majority of the heads are named after monsters in the game, suggesting that they may actually be the severed heads of said monsters.