7 Uses for Beast Cores

1. Beast Core Advancement

Beast Cores can be used as a method of advancing through levels, using the crystallized essence to improve the self. It's free XP. This does require a ritualist, and a ritual, but is a fast and effective mode of moving from low level/unranked to the mid tiers.

There is a fundamental flaw to using cores for advancement, and that is renders other methods of growth increasingly less effective. On a meta level, each core reduces the XP a character can gain by a percentage point, and this XP penalty will grow for each core they consume as XP, as well as the size of the cores consumed. A hearty warrior who chomps a weak core to push himself over a plateau will see something like a lifetime 1% XP reduction. A corresponding son of the nobility whouses his clout and coin to get the core from a slain dragon will have a rapid initial jump in power, but also something seriously impeding, like a lifetime 60% XP reduction from adventuring. 

In the DungeonVerse, there are generally two groups of people who use core advancement for their professions: people with zero interest in adventuring but want access to heroic powers and heroic level skills, and those who want heroic power and would rather pay money to get it rather than go into the dungeon to get it for themselves. In the former case, artisans and craftsmen want to break into the upper echelon of crafting, where their goods become magical and semi-divine, and their prices go from goat-headed copper pennies to dragon emblazoned gold coins. The latter are the coddled children of the wealthy and the powerful, who are generally expected to become powerful, but generally not exposed to the vulnerability and risk of the dungeon. Can't have the lone heir risk his life fighting a dungeon hydra and losing his arm and being scarred. Better to feed him up on a pile of cores instead.

2. Spell Components

Beast Cores are a common component in magic spells and rituals, with the most common role being magical 'oxidizers' or reagents, pumping essence into ritual circles.

3. Item Creation

Beast Cores retain a passive ability to draw in and purify ambient essence. They cannot grow or evolve anymore, being the magical equivalent of cut flowers put in a vase. The time frame is centuries, but the concept remains. This makes them highly valuable in creating magic items. The enchanting, etching, runes, and all the rest crafts a spell effect, and if a core is used, the item becomes self-poweringg. no

4. Raw Power Boost for Adventurers

Being an adventurer means managing regenerating resources like stamina, health, mana, and whatever. These replenish over time, or can be bolstered by the use of potions. Crushing a beast core can provide an infusion of these pool resources, though this is an exceedingly wasteful use of beast cores. It is also highly addictive, dangerous, and generates a euphoric effect. The only things that compare would be ripping a double rail of coke or dungeon dust. WHOOOOOOOO FIREBALL

5. One time Summon Item

A magic user specialized in summoning magic can use a Beast Core as a summoning focus and perform a one time summoning of the creature the core was taken from. Thus, a core taken from a basilisk would allow the summoning of a basilisk following the general summoning rules for level and duration of the spell. This doesn't cost the caster a spell, essence, or other ability, and the beast core shatters as it is consumed in the casting. High Level magic users who have mastered summoning can use a beast core as a summoning focus, but it does cost them essence, and they pay the cost of the spell and use the stone as a focus not the actual summoning device. This is rarely used because a high level summoner can likely already summon pretty impressive mobs, and the only cores they would use for this would epic and legendaries, which they would likely prefer to study and learn to caste rather than risk a spell failure that damages the core, or destroys it. Cores from legendary and epic monsters are rare. Or irreplaceable.

6. Awaken an animal into a beast

AKA lets Fullmetal Alchemist this regular animal by putting a beast core inside it. How the core is implanted is left to creativity. Eat it, surgery, a fusion spell, fired from a catapult. whatevs. The core starts regulating the flow of essence through the creature and then, it will start expressing itself, becoming a hybrid of the creature the core was taken from, and the host creature. It's power grows, its probably tortured and insane, but has cool or frightening magic powers.

Yes, a creature can be infused with more than one beast core.

You are a bad person if you do this on purpose.

Chimeras have done this to themselves, usually not on purpose. 

7. Template for Dungeon Creation

A dungeon core that is able to absorb a beast core gains the ability to start creating that beast as a mob, and can start using it to upgrade and modify the other mobs it has the templates for.