On June 13, 2016, Nikolas is revealed to be alive after falling out a window and he is on the same plane as Ava Jerome.
Nikolas and Ava were soon captured by Huxley Lynch, but they managed to escape. They were found in July on Cassadine Island by "Jason" and Sam, who knew all along that Nikolas faked his death. On July 11, after making out with him, Ava found out in Helena's diary that Nikolas had killed Helena by poisoning her tea. Dante, Lulu, Laura, and Kevin joined Nikolas, Ava, Jason, and Sam on Cassadine Island, but as they were all talking, Theo the fisherman walked in and took everyone hostage at gunpoint. On July 18, Theo revealed himself to be Valentin Cassadine, Nikolas' uncle and according to Helena, the most feared Cassadine. Valentin took Nikolas and Ava upstairs and forced Nikolas to give up all his fortune. As Valentin went to shoot Ava, Nikolas attacked him, but ended up getting shot and falling out the balcony. Nikolas was confirmed dead by Ava on July 22, though his body was never found.
On July 16, 2019, Ava asked a professional psychic to try and make contact with Nikolas, wanting to make amends for preventing Valentin from going to prison for Nikolas' murder. However, the psychic's attempts prove unsuccessful and she reveals that Nikolas is not in the spirit world, meaning that he may not be dead.
On August 13, another psychic named Chelsea Lambas warned Laura that her son was in danger and Laura got worried and called Lucky, who was still in Africa. After Lucky assured Laura he was fine, Laura accused Chelsea of being a fraud, but Chelsea said she was referring to her other son, which seems to be another hint that Nikolas could be alive.
It was later revealed in late August that there is a codicil to Mikkos Cassadine's will which could prove that Valentin is not the rightful Cassadine heir, although it is unknown if Jax and Hayden are working with Spencer or a not-so-dead Nikolas.
On October 31, Nikolas was revealed to be alive and was the one who was watching Ava. He was also the mysterious Cassadine ring man who kidnapped Cassandra Pierce in August 2018. He appeared as a masked man at the Metro Court Halloween Party and after running into Laura, Nikolas followed Ava to her gallery, prompting Ava to draw her gun on him and force him to reveal his face. Nikolas then revealed his face to shock Ava.
On November 4, it was revealed that Nikolas revealed himself to Jax months ago and asked for his help to find the codicil to Mikkos' will which could disinherit Valentin and return the Cassadine estate to the rightful heir, Nikolas. In return, Nikolas had promised Jax the shipping division of Cassadine Industries and he also promised Hayden a generous cut of the Cassadine fortune if she helped Jax search for the codicil. Jax and Hayden were upset with Nikolas since he took a big risk on returning to Port Charles but Nikolas was confident that no one will know he is alive until Valentin is destroyed. Jax reluctantly agreed to let Nikolas stay at his new house until the codicil is found. The next day, Laura went to Jax's house and Nikolas wanted to reveal himself but at the same time he couldn't risk it because he wanted to protect her from Valentin.
On December 4, Nikolas went to Shadybrook and paid a nurse to drug Ava. When Ava was drugged, Nikolas walked in and convinced her that he was in hell and Kiki was there with him which made Ava very upset. Nikolas claimed that she can make amends and that he and Kiki can be at peace if she tells him where Helena's portrait was. Afterwards, Ava gave in and told him she had it at her gallery and begged him to help Kiki and he promised her he would. Eventually, Nikolas left and Ava was shaken up and an emotional wreck after his visit. Unbeknownst to Nikolas, he left his ring under Ava's bed.
Nikolas met with Jax who said he was cutting his losses and ending their partnership since he did not sign up for any of the damage their mission was causing. Nikolas confessed that he staged the incident with Hayden so that she would leave town and also confessed that he knows where Helena's portrait was. Afterwards, Nikolas wore a mask and gloves when he broke into Ava's gallery and was about to look for the portrait but Valentin pulled a gun on him and ordered him to reveal himself. Nikolas assaulted Valentin in self-defense and took off.
On December 9, Jax told Nikolas he was done and he ordered Nikolas to leave but eventually Nikolas learned that he lost his ring. Nikolas hid as Valentin came by Jax's house and offered to partner up with him to find the codicil but Jax said no. Valentin accused Jax of stealing his inheritance and was trying to take him down so he could have Nina all to himself. When he left, Nikolas asked if he still wanted him gone or if he wanted to help get rid of Valentin but Jax said to find the ring. Nikolas remembered that it came off after his visit with Ava at Shadybrook but did not know that Nina took it.
On December 13, Nikolas was on Pier 17 but was hidden while Valentin and Charlotte were there. Charlotte saw Nikolas and said she knew who he was since she saw a picture of him on her mom's phone and Nikolas claimed that her mom knew him since he saved her life one time. Valentin saw her and asked who she was talking to and Charlotte claimed she wasn't sure. Valentin told her that she cannot talk to strangers because it's dangerous and they left to meet Nina. Jax and Carly met with Nikolas and Carly figured out that Ava knows he is alive since his ring was found in her room at Shadybrook and that Ava had a breakthrough overnight. Jax got a text saying that Ava was auctioning Helena's portrait that night which would give Nikolas a chance to get the codicil.
On December 16, Nikolas stayed in the back room while the auction was happening and he was upset when Helena's portrait suddenly burst into flames. Eventually, Trina Robinson spotted him but she didn't see his face. Trina threatened to call the cops on him but he left before she could. Nikolas met with Jax on the pier and said that he was going to destroy Valentin despite the portrait and presumably the codicil being destroyed.
Afterwards, Nikolas went back to the gallery and saw Ava aiming a gun at him. Ava confronted Nikolas about being alive all this time and demanded to know where he was for three years. Nikolas explained that he was pulled out of the water by a yacht crew and was recuperating in Athens. He explained that he learned there was a codicil that Mikkos left which returns the entire Cassadine estate to him and disinherits Valentin. Ava revealed to Nikolas that she has the codicil. She said she would give him the codicil because she wanted Spencer to get back everything that was rightfully his, but there were certain terms to be met.
After Valentin pushes Ava off the parapet at Wyndemere, Nikolas saves her and they get married at Nina's wedding which didn't happen that day. Nikolas and Ava made an arrangement that whoever cheats first in the marriage wouldn't get any of the Cassadine money.