42 cards where the cards have the “mardu” watermark

Alesha's Vanguard (Fate Reforged #60)
Alesha, Who Smiles at Death (Fate Reforged #90)
Ankle Shanker (Khans of Tarkir #164)
Arrow Storm (Khans of Tarkir #98)
Battle Brawler (Fate Reforged #63)
Bellowing Saddlebrute (Khans of Tarkir #64)
Bloodsoaked Champion (Khans of Tarkir #66)
Brutal Hordechief (Fate Reforged #64)
Butcher of the Horde (Khans of Tarkir #168)
Canyon Lurkers (Khans of Tarkir #105)
Chief of the Edge (Khans of Tarkir #169)
Crackling Doom (Khans of Tarkir #171)
Firehoof Cavalry (Khans of Tarkir #11)
Flamerush Rider (Fate Reforged #99)
Goblin Heelcutter (Fate Reforged #102)
Horde Ambusher (Khans of Tarkir #110)
Howl of the Horde (Khans of Tarkir #112)
Jeering Instigator (Khans of Tarkir #113)
Mardu Ascendancy (Khans of Tarkir #185)
Mardu Banner (Khans of Tarkir #224)
Mardu Blazebringer (Khans of Tarkir #115)
Mardu Charm (Khans of Tarkir #186)
Mardu Hateblade (Khans of Tarkir #16)
Mardu Heart-Piercer (Khans of Tarkir #116)
Mardu Hordechief (Khans of Tarkir #17)
Mardu Roughrider (Khans of Tarkir #187)
Mardu Runemark (Fate Reforged #107)
Mardu Scout (Fate Reforged #108)
Mardu Shadowspear (Fate Reforged #74)
Mardu Skullhunter (Khans of Tarkir #78)
Mardu Strike Leader (Fate Reforged #75)
Mardu Warshrieker (Khans of Tarkir #117)
Mardu Woe-Reaper (Fate Reforged #18)
Nomad Outpost (Khans of Tarkir #237)
Orc Sureshot (Fate Reforged #78)
Ponyback Brigade (Khans of Tarkir #191)
Timely Hordemate (Khans of Tarkir #27)
Valley Dasher (Khans of Tarkir #125)
Vaultbreaker (Fate Reforged #117)
War-Name Aspirant (Khans of Tarkir #126)
Wingmate Roc (Khans of Tarkir #31)
Zurgo Helmsmasher (Khans of Tarkir #214)

The message was clear to Sorin—time to search for something else.