A "tiger amulet" in Chinese history was used by the emperor to approve the deployment of military forces. One half would be kept safely, the other half would be kept by the field marshal (or equivalent commander-in-chief). When the emperor would deploy forces, the amulet would be handed over to the field marshal. If the two halves match, it would mean that the military forces have been approved to be deployed.
The Stygian Tiger Seal is presumably a tiger-shaped amulet of mysterious metal (found in Dusk-Creek Mountain). When activated, all walking corpses within a certain area comes under the wielder's absolute control.
The Stygian Tiger Seal’s powers were considerably greater than what its creator Wei Wuxian had imagined. He originally wanted to use it to assist him, but its powers were almost exceeding him. Moreover, it was bound to no one. This meant that if someone got hold of it, no matter who they were, whether they were good or bad, friend or foe, they were able to use it.
After the Seal had been forged, Wei Wuxian still held onto to it out of doubt and a deterrent to the rest of the cultivational world. He separated the seal into two halves, making it so that they could only be of use when they were put together, and never used without careful consideration.
Under Wei Wuxian, it had only been used twice, and both times caused great bloodshed. The first time was during the Sunshot Campaign. After using it for the second time, he finally decided to destroy one half of the seal. Before he could completely destroy the other half, the first siege of the Burial Mounds ended his life.
Following the siege, the other half fell into the possession of the Lanling Jin Sect. Xue Yang was a promising cultivator of Demonic Cultivation who could rebuild the other half of the Seal. Even though the recreated version wasn’t as powerful and couldn’t be used for as long, it could already result in terrible catastrophes.
Article: Yueyang Chang Sect, Xiao Xingchen, Nie MingjueAfter Xiao Xingchen found out the truth behind the massacre of the Yueyang Chang Sect, he went across three provinces and Xue Yang was still happily picking fights with others. Taking advantage of the Discussion Conference that was happening at the Carp Tower, Xiao Xingchen brought him over, explained the situation, and demanded severe punishment. The Lanling Jin Sect was thus forced to protect Xue Yang's life, since his value lay in recreating this weapon.