A Holy Avenger is a type of sword in D&D, sought after by paladins. Through a Holy Avenger, a Paladin may use his Paladin implemented powers with the sword's enchantment bonus on top of having a weapon that can harm almost any monster in hand. Such weapons are the stuff of legends across the multiverse and only one dedicated body and soul to his cause can hope to draw and wield it.
Holy Avengers have changed a bit over the editions even though the "Paladin's Toy" concept remained. It used to be a unique type of two-handed sword in AD&D, and a specific Longsword in 3e. In 4th Edition it became a weapon enchantment. Now it's back, as an official and Adventurer's League-legal magic item in the DMG for 5th edition, restricted to Paladins, with a Legendary rating and no re-sale cost. This is a VERY nice item for Charisma Paladins.