We are currently playing around with this in my Divine Right campaign:
HOLD THE LINE
When you take this action your speed becomes zero and you have disadvantage on dexterity saving throws UNTIL YOUR NEXT TURN. In return you can cleave thru an enemy, killing it instantly. IF YOU ARE ABLE TO ATTACK MORE THAN ONCE YOU CAN CLEAVE THRU MORE THAN ONE ENEMY. Any creature with resistance to melee attacks OR REGENERATION cannot be killed in this manner. You take attrition damage EQUAL TO 5X THE creature's CR (min. 1)
HOLD THE LINE
When you take this action your speed becomes zero and you have disadvantage on dexterity saving throws UNTIL YOUR NEXT TURN. In return you can cleave thru an enemy, killing it instantly. IF YOU ARE ABLE TO ATTACK MORE THAN ONCE YOU CAN CLEAVE THRU MORE THAN ONE ENEMY. Any creature with resistance to melee attacks OR REGENERATION cannot be killed in this manner. You take attrition damage EQUAL TO 5X THE creature's CR (min. 1)