Why Only Smite The Wicked?

Level 5, Eldritch Blast, Eldritch Smite Invocation

You may use an eldritch blast charge to instead use your eldritch smite on a successful hit. This may be done whether you have spells slots left or not. This smite starts at current spell level of damage, but is decreased by 1d8 per usage until only at 1d8 which is refreshed back to it's original full value on a long rest. This does not effect the regular usage of spell slots for smites. Those are always at full spell slot value.