Battle Dance

"The Blue Valor raced to their appointed location. We thought they were going to be slaughtered, charging the enemy head-on with light armor in a loose formation. They stopped about six feet short of the enemy, all as one. It was very impressive drill. Then they all started dancing. Their greatswords spun, between the two hundred mercenaries there were at least twenty forms going at once, all random, just a grinding, heaving mess of blades, feathers and tassels. One wing of the enemy was ordered to rush them, and it should have worked against a loose formation like that. What happened was... well, let's just say the enemy general went around, leading his men into the worst parts of the battlefield instead of sending any more into that."
~Krastop Holmegger, Balatar Sergeant of Lorical. 
The Battle Dance is a greatsword technique that leaves little opening for attack, at the cost of longevity in the fray. 
To use Battle Dance, a greatsword is required. Any creature within 10 ft. of the character must make a Dexterity Saving Throw at the beginning of each of their turns, or when they enter the space. The DC for this throw is 10+the character's Strength Modifier+their proficiency bonus. A failure allows the character to deal 1d8 slashing damage, plus their Strength modifier, to the creature. Additionally, if a creature ends its turn within 10 ft. of the character, the character may use their reaction to attack that creature.
For every 5 rounds in a single combat that this technique is used, the character must make a Constitution Saving Throw DC 9+the number of rounds the ability has been used in that combat. A failure means the character gains one level of Exhaustion and cannot use this ability again until they finish a short rest.