Post-Mortem Comeback

The Big Bad is dead, but there's something missing. The fans want to see more from him; the author has a few ideas left for him that didn't make it before he was killed; or the conflict hasn't entirely been resolved in your view. What can you do? Bring him back, without bringing him back.

A supposedly deceased villain can do this by previously laying one final failsafe to antagonize the heroes, or indirectly allowing their acts to force the heroes to hallucinate about him. A more unorthodox method allows the villain to transfer his memories, consciousness or soul into someone or something else, gradually allowing him to re-materialize in that new object or being like a parasite.

Compare Thanatos Gambit and My Death Is Just the Beginning. This may lead to Your Princess Is in Another Castle.

Anime and Manga

 * Bleach had one of the most disturbing examples, enough that it led to a Moral Event Horizon. At the end of his encounter with Mayuri Kurotsuchi, Szayelaporro Granz was devoured by Ashisogi Jizo, but he revived himself by implanting pieces of his Battle Aura into Nemu, allowing him to reform himself while inside her stomach. You read that right - he raped her.
 * A possible Trope Codifier has King Piccolo vomiting up an egg that contained his nearly-identical (appearance and personality) son, who eventually went into an Enemy Mine with Goku against Raditz.
 * Naruto had a particularly bad execution, with Orochimaru randomly appearing during the fight between Itachi and Sasuke.

Comic Books

 * In The Clone Saga, after the Jackal died, a post-mortem compulsion activates within Spider-Man's brain, causing him to try to kill whoever he loves most under temporary conscious mind control.
 * At the conclusion of a very long Spider-Man arc involving robots disguised as Peter's parents, we discover that the entire plan was set in motion by Harry Osborn (Green Goblin II) some time before his death. It gets even better because while Harry eventually forgave Spider-Man and moved on, the last time he was seen (prior to One More Day) was here, on a videotape he'd made, gloating over an enraged Spider-Man.
 * In The Incredible Hulk comics, after the Leader died (circa #345) and before he came Back from the Dead, he implanted his memories into a loyal follower who had a similar gamma-induced mutation as he.

Live Action

 * Airwolf once had this: Airwolf's mad creator had a program hidden inside it that nearly caused it to trigger World War III. The creator's been dead for quite a while, yet his revenge wasn't complete.
 * Buffy the Vampire Slayer: The Mayor, the Big Bad of season 3 who's killed at the end of the season, gets one final hurrah in season 4 when a contingency plan he set up lets Faith pull a Grand Theft Me on Buffy.
 * Harry Potter: Voldemort is incorporeal rather than dead, but he still manages to cause problems via a magic diary containing the personality of his teenaged self.
 * The Star Trek: Deep Space Nine episode Trials and Tribbleations features the Klingon from the Star Trek TOS episode Trouble with tribbles going back 100 years into the past to plant a bomb that will kill Kirk.
 * In the Star Trek: Voyager episode "Worst Case Scenario" (S3 E25), a highly adaptive hologram of enters the program and
 * In Stormbreaker (2006), the main villain has been foiled, but then Alex Rider remembers that he had a failsafe transmitter and so an extra 20 minutes of film time appears.

Troper Works
"Nireddel: "Be prepared for a post-mortem comeback.""
 * Soundscape is the Trope Namer, where Nireddel lampshades how a Humanoid Abomination pulled off the aforementioned unorthodox method.

Video Games

 * Square Enix had Sephiroth do this in Advent Children by focusing on his hatred of Cloud to keep from being assimilated by the Life Stream; while using Kadaj and his gang to find what was left of Jenova to bring about Hojo's reunion theory. Thereby allowing him to be reborn.
 * In Grim Fandango, the hero's rival Domino has been dead for over a year, but the good guys still have to contend with a deadly booby-trap he's laid for them when they finally get back to Rubacava to find the hiding place of their car. Dom's laid out an elaborate set of dominos all around the car that will trigger a bomb if they disturb it.
 * Mass Effect 1 had a variant, where Saren was reanimated into a husk.
 * Mass Effect 2 had the Derelict Reaper continuing to influence the crew that investigated its insides after it was disabled. Very, very Lovecraftian.
 * In Metal Gear Solid, Liquid's hand possesses Ocelot.
 * The final puzzle in Still Life 2 revolves around this. After finally identifying and killing the villain, our heroine rushes to save the hostage,

Western Animation

 * ReBoot had this. When Bob enters the core to save data and force a system restart, he encountered a program left behind by Megabyte, who at this point was launched into the Web (our resident Hyperspace Is a Scary Place). It's a simulation of Megabyte, which can self-replicate. All it does it taunt Bob.
 * In one of the most effective executions, we have Slade from the Teen Titans animated show. He was thrown into a pool of lava in the second season, but the dust in his mask left a chemical substance that made Robin see, hear and feel Slade, even when he wasn't there. It's also a subversion; Slade himself is resurrected by Trigon in the following season.
 * A variation of this occurs in Teenage Mutant Ninja Turtles (2003), where it is eventually revealed that Big Bad the Shredder keeps an online backup of his mind, in case his body is ever destroyed. After he was written off the series—exiled, but not killed—the writers had that backup take over the mind of the cybernetic villain Viral, allowing a new version of the Shredder to enter the fold.