Zagreus, the "first-born Dionysos," is a son of Zeus and Persephone who had been seduced by Zeus in the guise of a serpent. Zeus placed Zagreus upon the throne of heaven and armed Him with his lightning bolts, but Hera got jealous and had the Titanes dismember (and in some versions eat) Him. Zeus recovered only His heart and crafted it into a potion for Semele to ingest and thus give birth to Him as a reincarnation of the first. This myth seems to date back to a time before the version (of the birth) of Dionysos we have accepted into the general mythology of Hellenismos.

The idea of reincarnation probably dates back to the Iron Age (so around 1200 BC.). It enters the Hellenic stream of thought and philosophy around the 6th century BC, although there is mention of the theoretical subject in pre-Socratic philosophy. The ancient Hellenes most likely did not use the word 'reincarnation'; 'Metempsychosis' (μετεμψύχωσις) is a better word for the phenomenon they believed in. It is a philosophical term in the Hellenic language which refers to the transmigration of the soul, especially its reincarnation after death. The notion that the human soul enters another body upon death was widespread in Hellenic philosophy. The doctrine of transmigration is first associated with the Pythagoreans and Orphics and was later taught by Plato and Pindar. For the former groups, the soul retained its identity throughout its reincarnations; Plato indicated that souls do not remember their previous experiences. Although Herodotus claims that the Hellenes learned this idea from Egypt, most scholars do not believe it came either from Egypt or from India, but developed independently. Reincarnation in the form of transmigration was also a major feature of Orphism, and entirely unsurprisingly Zagreus featured heavily in Orphic mythology.

The Orphics were an ancient mystical cult with affinities to Indian religious systems. They believed in reincarnation and the possibility of liberation. Orpheus, the movement's legendary founder, is said to have taught that soul and body are locked together during life; the soul is divine, immortal and aspires to freedom, and during life, the body acts as a prison to the soul. Death releases the soul for a short while, but is then captured by another body until that, too, dies, and so the soul moves from body to body--both human and animal--until it can attain the highest good: liberation. In order to reach liberation, the Orphic way teaches to turn to God by ascetic piety of life and self-purification: the purer the life lived, the higher will be the next reincarnation, until the soul has completed the spiral ascent of destiny to live for ever as God from whom it comes. This is why their followers wore white, avoided places of death an birth (which are traditionally considered miasmic), and were vegetarians. Zagreus fits in perfectly with this philosophical concept.

So why Dionysos in the form of Zagreus? And why reincarnation? Zagreus was a Year-Daímōn, and reincarnation, as it appears, is the natural cycle of the world. The Year-Daímōn is a title given to any Theos or hero, whose death and rebirth are tied to the turning of the seasons. These scapegoats 'die' with the old year and are 'reborn' with the new year, setting in motion a cycle of renewal for the plant life that forms the basis of the Hellenic food supply. Dionysos is the prime Eniautos-daímōn, but Theseus, Herakles, Apollon, Odysseus, and Orpheus qualify as well, as They are--in some way--linked closely to death and rebirth, either from visiting the Underworld and leaving it, or experiencing a 'second birth' of sorts.

In this version of the myth, Dionysos is twice-born, hence his epithet 'Dimêtôr' (Διμητωρ, Of Two Mothers). Dionysos was considered a fertility God, but also closely related to nature's eternal cycle of birth and death. The ancient Hellenes considered the moment a plant--especially the grape--began to grow for the first time after being planted its first birth, and counted its second birth when it became laden with ripened fruit. As Dionysos is so closely related to to the grape vine, it was Dionysos Himself that was considered being born once from the earth and again from the vine.

Dionysos' cult focused heavily on this part of His mythology, and none more so than the Orphics. This earlier myth of His birth reaches back to  much older--possibly pre-Hellenic--'primitive' tribes which worshipped Dionysos or a God similar to Him with animal and human sacrifices which were torn apart, either before they were killed, or as a means to kill them. Most likely, these were the Thrakians. The God worshipped this way was also a God of life and death, and these sacrifices were conducted to ensure a good harvest in the coming year. This may also explain one of the retellings of the birth of Dionysos where He was torn apart by Titans.

How involved the worship of the Eniautos-Daímōn was, or how far-spread it was, is unclear. I suspect highly that the Orphic mythology and practice was focused on it, and while Orphism was a small mystery cult, it was widespread and even more widely known. Zagreus is one of Dionysos' wildest, most primal epithets and it underlay all believes about Him. It's an aspect of Him that bled into and out of Orphism and out of and into general worship. The two fed on each other, mixed and reformed--much like Dionysos Himself. Zagreus is a critical aspect of Dionysos to understand and yet, one that is most difficult to grasp. Perhaps this post has given you a glimpse.