Giant Slayer

From Dungeon Master's Guide, page 172. Found on Magic Item Table G.

Description
Weapon (any axe or sword), rare.

You gain a +1 bonus to attack and damage rolls made with this magic weapon.

When you hit a giant with it, the giant takes an extra 2d6 damage of the weapon's type and must succeed on a DC 15 Strength saving throw or fall prone. For the purpose of this weapon, "giant" refers to any creature with the giant type, including ettins and trolls.