So Herakles found Theseus in the Underworld after he went and finished his last labor of bringing Cerberus back.
What resulted of Theseus?
He was exiled from Athens and ended up on Skyros [Scyros] Island where he died falling off a high cliff, apparently murdered. His adventures sort of continue, to some extent, long after his death.
Going back in time to when Theseus’ father Aigeus [Aegeus] was still alive, Apollodorus tells us of a version of the story in which Aigeus’ father is not Pandion, who was King of Athens before Aigeus, but Skyrios [Scyrius], from whom the island of Skyros received its name. Either Pandion, for some reason, passed Aigeus off as his own son and made him his successor to the throne, or this was merely a malicious story cooked up by Aigeus’ jealous brothers in order to attempt to rob him of his legitimate claim to the throne. Whatever the case, Aigeus inherited estates on Skyros Island from someone, possibly Skyrios. We’ll come back to that.
Before his descent into the realm of the dead, Theseus had taken possession of the (underage) Lakedaimonian princess Helen, whom he left in the care of his mother Aithra [Aethra] in his kingdom of Attika [Attica], whose capital was Athens. While Theseus was trapped in the Underworld, Helen’s twin brothers Kastor [Castor] and Polydeukes [Polydeuces] came with an army to Attika in search of their little sister. After sacking the town where Helen was being housed, they retrieved the princess and took Aithra as a slave, making her into Helen’s maidservant.
Menestheus, a second cousin of Aigeus (if Aigeus really was a son of Pandion), was in Attika at the time of the attack of the Tyndaridai, “Sons of Tyndareus,” as Kastor and Polydeukes were popularly called. In fear for the complete destruction of Athens by the twins, Menestheus persuaded the city’s people to openly welcome the brothers back into Athens as their benefactors and deliverers. The Tyndaridai did no harm to anyone from there on out and rewarded Menestheus for his support by putting him on the throne of Attika and thereafter leaving the kingdom in peace.
Theseus then returned from his Underworld misadventure and was summarily exiled from Attika. He got a chance to make arrangements to send his two young sons Akamas [Acamas] and Demphoön to Euboia [Euboea] Island to be looked after over there by his friend King Elephenor of the Abantes. Theseus himself, meanwhile, set sail for Crete Island, whose king Deukalion [Deucalion] was his brother-in-law (Deukalion's sister Phaidra, now dead, was the mother of Akamas and Demphoön). Blown off course by the winds, Theseus landed, instead, on Skyros Island.
The version of the story supplied by Plutarch has it that Theseus was simply, and deliberately, heading for asylum on Skyros because, through his father Aigeus, he had ancestral estates at this location. Pausanias is the one who provides us with the account in which Theseus was on his way to Crete when he ended up, accidentally, on Skyros. In the latter case “he was treated with marked honour by the inhabitants, both for the fame of his family and for the reputation of his own achievements.” Either way Theseus died on this island, in both accounts having been murdered by Lykomedes [Lycomedes], the king of the Dolopes who lived on Skyros, although in Plutarch there is an attempt to vindicate Lykomedes by offering an alternate ending to the story.
Pausanias offers no details about the death beyond saying that it was contrived by Lykomedes because of Theseus’ positive reception by the islanders of Skyros. Plutarch implies that Theseus mistakenly assumed that the people of Skyros were his friends, on top of which he had the advantage of a claim to territory over there. Upon his expulsion from Attika, Theseus applied to King Lykomedes a request for his lands to be restored to him, although it is also said, Plutarch tells us, that Theseus asked for Lykomedes’ help against those who had exiled him from Athens. Either Lykomedes was fearful of Theseus’ renown or he was incidentally more favourable to Menestheus, and so, on pretence of showing Theseus the requested estates from an elevated vantage point, he shoved the Athenian exile off a cliff, thus killing him.
Some, however, say that he slipped and fell down of himself while walking there after supper, as was his custom. At the time, no one made any account of his death, but Menestheus reigned as king at Athens, while the sons of Theseus, as men of private station, accompanied Elephenor on the expedition to Ilion [i.e. the Trojan War, which broke out a few decades after Theseus’ death].
Menestheus led the Athenian forces against Troy and after the war ended he never made it back to Attika, instead landing on Melos Island to become its king there, where he later died.
The cause of the war was that the Trojan prince Paris had abducted or seduced Helen, who by this point had become Queen of Lakedaimon. Ironically it was Akamas who, together with Diomedes, was sent on an ambassadorial mission to Troy to attempt to get Helen back peacefully. The mission failed and Greece ended up going to war against Troy. The more famous account of the embassy, in the Iliad, ascribes the failed embassy to Helen’s husband Menelaus and to Odysseus.
As Helen was finally being restored to Menelaus at the end of the war, Akamas and Demophoön were able to retrieve their grandmother Aithra from servitude. They took her back to Athens where they recovered their father’s throne, now that Menestheus was gone for good.
Roughly seven hundred years later, in 490 BC, when the Persians attacked the Attikan city of Marathon, some of the gods, including Pan, Athena and Herakles, are said to have aided the Greeks against the “barbarians,” so that, under the leadership of General Militiades, the Greeks won the Battle of Marathon. One of the phenomena reported from the battle is that Theseus was seen fully armed leading the charge against the Persians. How they recognised him doesn’t seem to be explained but it was understood that his shade rose up out of the Underworld in order to assist his fellow citizens in hand-to-hand combat. For this service, Theseus was worshipped as a hemitheos, “half-theos” or “demigod.”
25 years after that, Miltiades’ son Kimon [Cimon], who was also an army general, ravaged Skyros. Pausanias interprets this as revenge for Theseus’ death, after which achievement Kimon carried Theseus’ bones back with him to Athens. At the end of his Life of Theseus, Plutarch elaborates on this further, saying that:
[I]n the Archonship of Phaidon, when the Athenians were consulting the oracle at Delphi, they were told by the Pythian priestess to take up the bones of Theseus, give them honourable burial at Athens, and guard them there. But it was difficult to find the grave and take up the bones, because of the inhospitable and savage nature of the Dolopes, who then inhabited the island [at least since the time of their king Lykomedes]. However, Kimon took the island … and being ambitious to discover the grave of Theseus, saw an eagle in a place where there was the semblance of a mound, pecking, as they say, and tearing up the ground with his talons. By some divine ordering he comprehended the meaning of this and dug there, and there was found a coffin of a man of extraordinary size, a bronze spear lying by its side, and a sword. When these relics were brought home on his trireme by Kimon, the Athenians were delighted, and received them with splendid processions and sacrifices, as though Theseus himself were returning to his city. And now he lies buried in the heart of the city, near the present gymnasion [gymnasium] and his tomb is a sanctuary and place of refuge for runaway slaves and all men of low estate who are afraid of men in power, since Theseus was a champion and helper of such during his life, and graciously received the supplications of the poor and needy. The chief sacrifice which the Athenians make in his honour comes on the eighth day of the month Pyanepsion, the day on which he came back from Crete with the youths. But they honour him also on the eighth day of the other months, either because he came to Athens in the first place, from Troizenos [Troezen], on the eighth day of the month Hekatombaionos, as Diodoros the Topographer states, or because they consider this number more appropriate for him than any other since he was said to be a son of Poseidon. For they pay honours to Poseidon on the eighth day of every month. The number eight, as the first cube of an even number and the double of the first square, fitly represents the steadfast and immovable power of this god, to whom we give the epithets of Asphaleios1 and Gaieokhos.2
1. William Smith’s 1886 Dictionary of Greek and Roman Biography and Mythology says of this epithet that it describes Poseidon “as the god who grants safety to ports and to navigation in general.”
2. “Holder/Embracer/Encircler of Gaia [Earth],” translated as “Earth-Stayer” in Bernadotte Perrin’s 1914 translation of Plutarch’s Lives.
Well it depends which source you want to follow.
If it is Plutarch, then he is dead.
Now in Euripides' tragedy "Herakles", the hero frees Theseus from the Hades From what I recall, the events are in Thebes, where Lycus usurped the throne and acted as a tyrant. Herakles' wife is there and Lycus is trying to kill her and her sons but as Herakles is in Hell she cannot defend herself. Finally Herakles comes and kills Lycus, but also - being enraged - his wife and sons.
Now Theseus, which as Herakles is a good guy spending his time killing bad people, knowing the usurpation of Lycus comes to Thebes to kill him, but learn Herakles did it and also his family. He cheers him and they both go to Athens.
That is from memory only. But it is probably remotely accurate. Think you can find the tragedy in any online library like Gutenberg.
I have read in a book (I forgot the name but it had to do with the 12 tasks Herakles had to do) and it said that in one task Herakles ran across Theseus, as stated in your question, and when asking Hades to get Cerberus, he asked if Hades could free Theseus and Hades agreed. Theseus and Herakles then lived a normal life.