When wielding a melee weapon, as an Attack action you can choose to lock blades with an enemy creature holding a melee weapon by making contested attack rolls against the target. If you win, the enemy creature's weapon is "blade locked" by you. If they make an attack with that weapon or perform an action that causes them to drop that weapon, they incur an opportunity attack from you. Furthermore, any other actions they attempt with their offhand are performed at disadvantage.
As an action at the start of their next turn, the blade locked creature can attempt to wrest control of the blade lock and turn it to their advantage. They may also choose to use the disengage action to leave the blade lock without incurring the opportunity attack.
The target of a blade lock advantage on its contested check if it is larger than the attacking creature, or disadvantage if it is smaller. Creatures have advantage on blade lock checks when wielding a versatile or two-handed melee weapon, or if they use both dual wielded weapons during the lock.