TigerClan is a minor, but pivotal antagonistic faction in Erin Hunter's Warriors book series.
It was a combination of ShadowClan and RiverClan with the goal of taking over the entire forest territories. It was formed and named by Tigerstar, who took inspiration from the mythical TigerClan from the Clan legends.
TigerClan served as the secondary antagonistic faction of "The Darkest Hour", the final book of the original first arc, the main antagonistic faction of the super edition "Leopardstar's Honor" and a posthumous antagonistic faction in "The New Prophecy" arc, graphic novel "A Shadow in RiverClan", novella "Blackfoot's Reckoning" and super edition "Firestar's Quest".
TigerClan is the combination of ShadowClan and RiverClan, led by both Tigerstar and Leopardstar (although Tigerstar eventually took over the whole leadership). The conjoined Clan is formed at the beginning of The Darkest Hour, and spends most of the book attempting to assimilate the cats of WindClan and ThunderClan in order to completely rule the forest.
ShadowClan and RiverClan agree to form an alliance in order to become stronger (and likely also due to Leopardstar's attraction toward Tigerstar). When this is announced at the Gathering, Tigerstar then requests Firestar and Tallstar join their Clans with his in order to unite the clans. The two leaders refuse, with Firestar knowing Tigerstar won't share land and prey equally as promised. However, before the ThunderClan leader can reveal Tigerstar's crimes, a thunderstorm interrupts him, bringing the Gathering to a premature close.
Later, TigerClan attacks WindClan to pressure them into joining, with Tigerstar killing an apprentice named Gorsepaw in front of the Clan to serve as an example of what the WindClan cats can expect should they continue to refuse.
Later still, TigerClan holds Mistyfoot, Stonefur, Featherpaw, and Stormpaw prisoner for being half-clan cats. When Firestar, Graystripe, and Ravenpaw sneak into RiverClan camp to spy, they witness a meeting in which Tigerstar, sitting atop a pile of prey's bones, brings the starved prisoners out and orders Stonefur, the former RiverClan deputy, to prove his loyalty by killing Stormpaw and Featherpaw. When the RiverClan tom refuses, Tigerstar orders Darkstripe and then Blackfoot to kill him. Once Stonefur is dead, the remaining three prisoners are sent back to their den, where they are later rescued by the three spies.
Finally, when TigerClan meets with ThunderClan and WindClan at Fourtrees, Tigerstar demands one last time for them to join together. When the remaining Clan's leaders refuse once more, Tigerstar reveals his alliance with BloodClan and orders the rouge cats to attack. When BloodClan's leader Scourge refuses and Firestar tells all of the gathered cats of Tigerstar's crimes, Tigerstar lashes out at the leader of BloodClan before being slashed open from chest to tail, losing all nine of his lives one after the other. After their founder's death, TigerClan disbands permanently.
TigerClan on the Warriors Wiki
The Dark Forest