The latest unearthed arcana has two invocations of similar features. The Curse Bringer and the Mace of Dispater, the latter dealing force damage instead of slashing and knocking the target prone instead of reducing the speed. I am not sure whether you have to use the spellslot if you want to reduce the speed of your target.
When you hit a creature with this weapon, you can expend a spell slot to deal an additional 2d8 slashing damage to the target per spell level, and you can reduce the creatures speed to 0 feet until the end of your next turn.
You have to use the spellslot for the damage, but it seems that the reduction in speed is completely independent from this requirement, which would be very powerful without saving throw.
Best Answer
You must use the spell slot to trigger the ability.
Let's break it down.
When you hit a creature with this weapon:
You can expend a spell slot:
The second clause of the invocation, "and you can reduce the creature's speed to 0 feet" is not an independent clause. It is dependent on the first part of the sentence, which says "you can expend a spell slot [to do the following]." The second "you can" indicates that reducing the target's speed to 0 is optional when you spend the spell slot, not that it is separate from spending a spell slot.
Unfortunately, the copy for this ability has some poorly placed commas which creates a bit of ambiguity. I would hope the wording for these abilities would be clarified were they to be printed in an official source.