Echo Collector

When a spellcaster you watched wield magic dies in a battle where you dealt it harm, a final echo of its power lingers in the air like a fading memory. You seize this echo as a one-use version of a spell the creature knew, chosen by the DM. This spell uses its own spell attack bonus and save DC, as a spell scroll does. You cast it once as though it were a personal spell scroll usable only by you. You can hold only one captured echo at a time, and if you cast it or finish a Long Rest, the echo vanishes forever.