Summoner Spell: The Summoning Stone
Groups of Summoners can enchant a magic conducting stone with energy, such energy calls forth dark entities you can fight for experience. Summoning Stones can also be used in sieges as ammunition for castle sieges. Beckoning forth monsters to attack friend or foe alike.
Should you defeat the energies attracted to the summoning stone, you create a magical item that is charged with their dark energy...which is volatile...explody, volatile.