Bound Siltstone
This palm-sized disc of layered siltstone was pressed and fired in the ruins of an arcane archive that collapsed inward during a catastrophic surge, entombing every spell ever recorded within its strata. Scholars who have examined it claim each dark band represents a different school of magic, compressed into mineral memory over centuries. A caster who holds it while a mind goes quiet can feel the pressure of accumulated endings pushing outward against their palm.
Mechanics
When you reduce a creature to 0 hit points with a spell, the focus captures a fragment of that creature's arcane signature, which you can expend to empower the next spell you cast before the end of your next turn.
Sediment of Endings
When you reduce a creature of challenge rating 1 or higher to 0 hit points with a spell cast using this focus, a mote of captured resonance settles into the stone. The next spell you cast before the end of your next turn deals an additional 2d8 force damage to the first target it hits or affects. Only one mote can be stored at a time, and it dissipates harmlessly if not expended by the end of your next turn.
DM Notes
Balance consideration: the 2d8 force damage rider is meaningful but gated on a kill condition, which means it rewards decisive spellcasting rather than passive benefit. In encounters with many low-CR enemies the effect can chain rapidly across mobs, potentially adding 2d8 to nearly every spell in a long fight. Consider that this is working as intended at rare tier but monitor players who build for AOE kill-fishing to proc it repeatedly. The CR 1 floor prevents trivial activation on minions or summons. The mote dissipates end of next turn, so players cannot bank it between encounters. Compatible classes reflect all primary spellcasters who routinely use an arcane focus and cast offensive spells.
Generated with: Level 5 · Arcane focus · Heroic · Rare