Welcome to EP.
Even though it seems not to fir your description, the
circuit described here should be just right for you.
Replace switch S1 by a direct connection to 12 V.
The LED will fade in upon turning on the 12 V.
The LED will instantaneously turn off upon turning off the 12 V.
You may need to adapt component values for other supply voltages or an other number of LEDs.
You may speed up discharge of cpacitor C1 by adding a diode in paralle, anode to 0 V, cathode to the capacitor's "+". This will arm the circuit for the next power-on sequence much faster.