In the Myth of Perseus, the titular hero killed the monstrous gorgon named Medusa, bringing her snake-covered head back to King Polydectes as a gift for a large banquet. Eventually, Perseus used the powers of the decapitated head to kill Polydectes once he learned about the man’s violent advances towards his mother Danae.
Similar situations occur within other myths. Tales depicting great heroes slaying various beasts, taking a part of said beast as a trophy or a gift for someone, then use that morbid item as a weapon (usually with mystical properties). Heck, there’s an entire video game franchise based around the concept: Monster Hunter.
Taking that into account, I think we can incorporate the idea into Dungeons & Dragons, allowing for the creation of a new kind of magic item: MONSTROUS ITEMS.
Unlike other magic items, monstrous items can take just about any form. They can be potions, wondrous items, weapons, etc. The only defining feature of a monstrous item is that it was made from the body parts (or fluids) of a particular creature. The item usually grants the wielder access to one of the base creature’s key abilities, but in a more limited fashion.
The key to creating a memorable monstrous item is similar to creating a memorable magic item: make sure the item is cool, yet thematic. For example, let’s continue to use Medusa’s Head as an example. The item allows you to utilize the creature’s petrification abilities, most likely by thrusting the head towards a creature, forcing them to look at it. Not only is the ability to possibly turn an enemy to stone cool, but it fits the flavor of the item.
I also suggest making monstrous items a rare sight within your campaign world. This will make acquiring one more special. It will also make encountering certain creatures more intriguing as well. You can explain the rarity away pretty easily due to the practice of turning the body part of a dead monster into an item could be seen as a little morbid, plus the magic needed to preserve the creature’s ability might be less common than other arcane skills.
Finally, I would also give these kind of items a weird cost or drawback. This might be my own preferences coming into play, but I find magic items that have a distinct quirk more interesting because they can lead to some cool moments within the story of the game. They also can make players think twice about using an item, possibly leading them to think of more inventive solutions to problems.
I’ll use the aforementioned Medusa’s Head as an example of what a monstrous item might look like mechanically:
MEDUSA’S HEAD (Monstrous Item, Rare; Requires Attunement) ~ This item appears to be the decapitated head of a maiden with a nest of vicious snakes as hair. The eyes and mouth upon the head seem to be shut tightly. You may use an action and point the head towards a creature within 30 feet that can see it. The eyes upon the head open wide, unleashing its cursed gaze, forcing the creature to make a DC 15 Constitution saving throw.
Creatures that fail the saving throw by 5 or more are instantly petrified. Otherwise, creatures that fail the save begin to turn to stone and are restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.
You must be careful when wielding this item because if you are ever made to look at it while the head’s eyes are open, such as using it before a reflective surface, you will have to make the DC 15 Constitution saving throw as well. You can spend a bonus action to quickly advert your eyes, allowing you to avoid the saving throw, but suffer disadvantage on Wisdom (Perception) checks until the beginning of your next turn.