Polearm Master Feat

You can keep your enemies at bay with reach weapons. You gain the following benefits:

Prerequisite: None

• When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear, you can use a bonus action to make a melee attack with the opposite end of the weapon; this attack uses the same ability modifier as the primary attack. The weapon's damage die for this attack is a d4, and the attack deals bludgeoning damage.
• While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.

Source: PHB'14, page 168