I'll Tell You a Story

Clarke tells Landon about Malivore. People were suffering under constant attacks by supernatural creatures. A witch, werewolf, and vampire came together and created a golem to kill and absorb supernatural creatures, except for witches, werewolves, and vampires. The golem, known as Malivore, was so successful it became self-aware and refused to allow its creators to destroy it by making a deal with humans to stop the "triad". Malivore attempted to create more golems, but they were flawed. Clarke reveals he is Landon's half-brother, as he is one of Malivore's "failed" creations and he convinced humans to perform a ritual turned Malivore into its current state. Humans formed the Triad, which turned into Triad Industries. They watch over Malivore, capture supernaturals and throw them into the puddle. Clarke wants to restore Malivore by bringing it the three original objects used to trap it. He reveals Landon is Malivore's son. He convinces his "little brother" to find the final key. Lizzie tries to restore her relationship with Josie. Rafael is suffering blackout because of Hope's potion. Emma takes a sabbatical to convince Alaric to ask for help. He asks Dorian to return. Veronica learns of Clarke's betrayal and sends Triad Agents to attack the school.

Plot summary from Wikipedia (CC BY-SA 4.0). Source