Undying Army Commendation

Undying Army Commendations are rewarded by various quests and world quests. When a quest that grants an Undying Army Commendation is completed, you automatically receive reputation with the Undying Army.

Source
Maldraxxus world quests (+175 reputation):
 * A Few Bumps Along the Way
 * Deadly Reminder
 * Gettin' Even

Ta'lan the Antiquary weekly quests (+500 reputation):
 * A Valuable Find: Plaguefall
 * A Valuable Find: The Other Side

Finder Ta'sul weekly quests (+500 reputation):
 * Trading Favors: Plaguefall

Trivia

 * Early in the Shadowlands beta, the Trading Favors quests awarded Maldraxxian Armaments instead.