I would revise the damage as having it be 3d8 - 5 opens the opportunity for it to not deal any damage on the failed save. (if one is decently lucky they will take less damage on a fail than a success.)

It also would probably work better with either some re-balancing or changing it to a higher level spell.

I fixed your spell 

Aard's Forceful Fist

1st level evocation

Casting Time: 1 Action (ritual)

Range: Self

Components: V, S, M (A bandage covered in chalk)

Duration: Concentration, up to 10 minutes.

Classes: Artificer, Cleric, Ranger, Paladin, Sorcerer, Wizard

A cold breeze begins to swirl around your fist, filling you with vigor. The range for your unarmed strikes increases by 10 feet. When you attack with an unarmed strike, make a melee spell attack. On Hit: the creature takes 2d8 Force damage. If the target is within your normal unarmed strike range, the damage is 3d8 force damage and the creature hit is forced to make a Strength saving throw. On a failed save: it falls prone. If it succeeds it does not fall prone.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the Force damage increases by 1d8 for each slot level above 1st.

I will make it 2 or 5 level spell For balance