So I've been reading up on some old Greek stories lately, and I came across the story of Thanatos. If I'm correct, he is supposed to carry the souls of the dead to the Underworld as they died. But even in ancient times, hundreds if not thousands of people died each day. Could Thanatos really carry the souls of each of them one by one to the Underworld (I would suspect not since it would take far too long)? Did he have super speed or was something else going on?
A long time ago, I read where the waning and waxing of the moonlight is from the collection of dead souls. When the moon is completely dark/full of souls, they begin to be ferried away and the Light gradually reappears. Eerie, and I still think of it from time to time when noticing the moon phases.
Just found something akin to the above, tho they think the soul is the Light. I remember a saying, "What you fear is the darkness of your own soul." So, my vote would be for the dark moon to be souls.
At all events, the moon is always understood as the receptacle of the souls of the dead.
They migrate to the moon after death, and the moon gives birth to the souls in the sun. She first gets quite full of dead souls—that is the pregnant full moon—and then she gives them to the sun, where the souls attain new life (a Manichean myth).
Psychopomps (from the Greek word ψυχοπομπός, psychopompós, literally meaning the "guide of souls") are creatures, spirits, angels, or deities in many religions whose responsibility is to escort newly deceased souls from Earth to the afterlife. Their role is not to judge the deceased, but simply to guide them. Appearing frequently on funerary art, psychopomps have been depicted at different times and in different cultures as anthropomorphic entities, horses, deer, dogs, whip-poor-wills, ravens, crows, owls, sparrows and cuckoos. When seen as birds, they are often seen in huge masses, waiting outside the home of the dying.
Ancient religion Classical examples of a psychopomp are the ancient Egyptian god Anubis, the Greek ferryman Charon and deities Hermes and Hecate, the Roman god Mercury, the Norse Valkyries, and the Etruscan deity Vanth.
"Thanatos is the god of death, in fact, he is Death, but he did not ferry souls to the underworld, that was Charon’s job. Thanatos was more like the Reaper, when someone’s time was up he would go to them, touch them, and kill them. he also kept people from returning to life from the underworld (like a life/death border patrol)."
In every story featuring Thanatos as a major character his primary role does in fact seem to be the transportation of the dead, via some means or another, to the Underworld, and it does so happen that each of these stories is about a singular dead or dying human.
Taking into account, as your Question does, the sheer numbers of humans who die daily, it would indeed logically require Thanatos to "have super speed" or otherwise be everywhere at the same time, in some way. Assuming that to be the case, it sounds to me like this would make him immensely powerful, perhaps even with limitless power, or at least very few limits to these powers.
Death is a Loser
However, the two best-known of these Thanatos tales are also about how easy it is to overcome this god, at least temporarily. We do not have the fine details of the first story because it comes from a now-lost play by Aeschylus, about the notoriously cunning Corinthian king Sisyphus, who angered Zeus, as a punishment for which the king of the gods sent Thanatos to put an end to him.
Using some sort of trick, however, Sisyphus managed to bind Thanatos for a while, within which time humans stopped dying. The war-god Ares then came and rescued the god of death, who was then able to complete his task of killing Sisyphus. The particularity about no one dying during Thanatos' captivity suggests that the death-god is actually neither omnipresent nor nigh-omnipotent, as though he is in fact a localised phenomenon, able to attend to people in only one place at a time.
The second story takes place a few generations later and also comes from a play, namely Alcestis, by Euripides. In it, Thanatos arrives on the scene to take Queen Alcestis of Pherae away to the Underworld, but in a wrestling-match, which happens off-stage and doesn't seem like much of a competition for the victor, the great hero Heracles [Hercules] beats the god of death and snatches Alcestis from him to take her back to her husband Admetus.
A conversation which passes between Apollo and Thanatos makes it fairly certain that the god of death is conceived of here as bearing off the dead one by one to their eventual home under the earth. One might say it is surprising how easily Thanatos is vanquished here, but to be fair Heracles is at least as strong as his father Zeus, and holds his own in battle quite well against huge monsters, giants, and certain other gods who are nearly as powerful as his father.
Who is Providing Transport?
The role of the god Hermes Chthonius, "Hermes of the Underworld," appears to duplicate Thanatos' apparent function as the bearer of the dead to the nether realm. Hermes is rather distinctively known to be the Psychopompus, "Guide of Souls," of Greek mythology, envisioned as leading a train of dead humans to the rivers of the Underworld where he hands them over the boatman Charon to ferry them from there to the palace of Hades, the ruler of the Underworld, for judgement as to their future accommodations.
The stories of Sisyphus and Alcestis, however, make it sound rather like it is actually Thanatos who is the Psychopompus. Other mythography ascribes this position to other entities. From Hesiod's Shield of Heracles, for instance, we learn about how Thanatos existed as, essentially, multiple female versions of himself called the Keres. In the Theogony, also by Hesiod, both the Keres and Thanatos are the fatherless offspring of Nyx, the primaeval goddess of night.
Theoi.com interprets the difference between Thanatos and his sisters as the former being the personification of non-violent death while the Keres represent violent death. In Homer's Odyssey 14.207 Keres are described explicitly as carrying someone away "down to the house of Hades", suggesting to us further that Thanatos and Hermes are not the only couriers of the dead, unless we take these different references to be simply conflicting and contradictory ideas about the transportation system for the deceased.
The Blood-Stained Visage of Mortality
The Roman authors Seneca and Statius describe Mors, the Roman version of Thanatos, quite vividly as more of an all-powerful force of nature, calling him gory and insatiable, hovering over battlefields with his mouth wide open in order to gobble up the dead and then vomit them back out upon reaching the Underworld with them.
In Seneca's Hercules Furens, Mors appears to be of immense size, "pale-faced with greedy teeth," while in the same author's Oedipus, it seems as if mortals cannot die fast enough for him to devour and crowd them in front of Charon before he hungrily rushes back to the upper world to fill himself up some more humans.
In Statius' Thebaid he is even the one who cuts through the threads of life which are spun by the goddesses of fate for each human being, and once the dead are standing before Hades, the death-god enthusiastically tallies up the deceased for the Underworld king.
These depictions very much seem to envision a gigantic being who moves with blinding rapidity, all of which make it difficult to reconcile him with the somewhat comically pathetic character in Aeschylus and Euripides. Either it just does not make sense to have Thanatos take up time he does not have calling upon the dying one by one, whereby we have contradictory material to work with, or we could say that Thanatos is not in fact the sole guide of the dead to the house of Hades, since we do also encounter other deities performing this task.
Facing the difficulty of how Thanatos is able to fulfill the role assigned to him in the lost Sisyphus play and in Alcestis, we at least have the words of Odysseus to fall back onto: "it is hard for mortal men... but with the gods all things are possible" (Odyssey 10.306), for whatever that is worth.