The Nether Emblems are the three items that create the Nether Saber, the only weapon that can defeat Nox Decious. In "Three Objects make a Cool Weapon," Mario and the others learn that they need to find these three objects to pierce through Decious' defenses. They are the Shield, the Hilt, and the Cross.
The Emblems were used by Merlin to create the Nether Saber at his table to defeat Nox Decious. After their battle, he split the power of the beam sword into 3 separate items so that no person could control its power at once.
Through the ages, they split apart from each other.
The Ronfair Dragon, commonly referred to as the Shield throughout the series, is an ornamental shield that has Nox Decious' symbol on it, given to him by an acquaintance during his youth. Merlin used to hide it for Decious to find as an exercise of patience and skill.
When Mario, Luigi, and their best friend try to recover the Nether Emblems, Donkey Kong and Brock are sent to search for the shield. In the Orchard, Donkey Kong spots it in the tall grass. They bring the shield to Mario at the campfire. Mario keeps it until it is united with the other emblems in "The Most Epic Battle EVER."
The Hilt is, as the name states, the hilt of the Nether Saber. When Merlin first created it, the saber was presumably able to function without needing the cross or the shield.
Link and Ness quest for this in the episode "Is it "Kung Pow" or Stupid Mario?". Johnny Cyclops brings them to Bowser's hideout, where Link recovers it. They then bring it to Mario, who keeps it safe.
The Cross is an ancient metallic cross that once belonged to Merlin and Nox Decious' mother. It was apparently given to Decious after her death, but he lost it later on. Unbeknownst to Nox, it was presumably in Merlin’s Beam Sword, The Nether Saber.
The cross was buried with Decious at the location, which later became the campfire. Here, Waluigi finds and removes it from its case. He keeps it intending to give it to Walgina (not knowing what it really is), but Mario takes it from him on the soccer field. It remains with Mario until he loses it in the Orchard.