Biting Blade

One edged weapon that you can see  becomes incredibly sharp.  The next two successful hit with that weapon within the duration of the spell deals damage dice one category larger: d4 becomes d6, d6 becomes d8, and so on.  Note: d12 becomes 2d8.  The damage the weapon deals is considered magical for purposes of resistance

On the second successful hit, the spell ends.

When cast at higher levels the number of hits the spell applies to increases by 2 per level above 1st.

e.g. at 4th level, the spell would expire after 8 hits or 1 minute.