Iaido Class?

[Ogrillian]

Iaido feat
Only works while wielding Katanas, LongSwords, Scimitars, and ShortSwords (great swords are to heavy, rapiers to thin of a blade, and daggers have to short of a blade to use this)
• Requirs 14 Dexterity
• You draw and sheath your weapon with each attack
• Iaijutsu — During your Attack Action you may Concentrate and “store” your Attacks, then unleash them as a Bonus Action attack on the same turn, adding the extra damage to a single weapon attack. Also when you “store” each attack you reduce your Bonus Action attack’s Crit range by 2 and add 1d4 slashing damage per attack stored.

You may store a number of attacks equal to your Dexterity modifier per short or long rest.

ie A lvl6 fighter with 17Dex and a long sword attacks once and stores his extra attack, his bonus action attack will have a crit range of 18-20 and deal 2d10+3+1d4 slashing damage

A shadow monk with this could do wonders as well